morriswalters - I hope I haven't caused you offense. Perhaps we will discuss it further one day. And by "all of the fey" you mean some I think...at that particular battle several characters and creatures were described specifically using other weapons (e.g trolls with clubs, fey with spears, Winter Lady and Queen etc.) But really we are talking about humans, not creatures of magic. Specifically Wizards - who largely rely on magic for combat. My memory of the humans that use swords are (the 4 knights of the cross - butters only technically, Nicodemus himself - I don't recall ANY other Denarians using swords as most rely on their Fallen's monster form, Lloyd Slate and Fix - as Winter and Summer Knight respectively) But most don't (the Denarian squires use guns, Kincaid doesn't use any swords - though he isn't strictly speaking human, none of the cops other than Murphy, None of the Black Court Vampires, only some of the Reds - they also use obsidian axes and blades rather than traditional swords, the White Court Vampires do - but they also use guns and explosives just as much and are considered the most modern of the Vampire Courts, Charity uses hammers, Molly doesn't use swords, none of the priests, not Marcone or his people apart from Gard who is probably a Valkyrie etc.)

Harry likely learned fencing like everyone else does - in a class. Fencing classes are everywhere. But the only teach fencing as a sport, anyone old school (like Morgan or Luccio or Grevane etc.) would have learned it not for sport but for defense. Warden's likely learn it from each other. He has fought twice I believe as he normally likes to express Earth Magic through his sword cane - primarily electromagnetism. He has used the blade though, I will have a search around for the text. But he does it rarely, as like most wizards he relies heavily on magic in combat. Sword users that have been described, especially humans are in the minority in the series. Don't get me wrong, I love sword fights and duels. Great stuff to watch, I have seen many acted out and been to many fencing competitions not to mention watching many online. I have actually seen the JB proposal which was very sweet. But none of this means that Swords are common to humans (especially magic humans) in the series.

Maz - I think you are confusing Harry piercing veils with magic and his powers of deduction. He recognised Talos and Korrick through process of elimination (he knew Auror was behind the theft of power, meaning her people would be the one's with her but he didn't "see through" the veils sphysically. He would have had to use his Sight). I believe in the Mavra case you are referring to is when he first meets her at Bianca's ball. In that case he used his magical "sense" to work out where an invisible person was - but again did not "see" through the veil. He would have had to call upon his Sight. Which he does to Ariana during the duel with her - but any practitioner with the Sight could do such a thing. It is hardly unique to Harry. He is frequently missing veils actually (Cowl and Kumori snuck up on him, Elaine and Korrick catch him straight off guard despite almost being on top of him, Martha Liberty was only feet from him an Ebenezar yet he wasn't even slightly aware of her and Listen's to Wind etc.)

Would you recognise her? Even with a physical disguise that obscured her body shape and face, and a magic voice distorter?  Its conjecture anyway. The fact is Harry does not recognise Kumori. We can only speculate as to why. Perhaps he wouldn't recognise Faith or Jenny after so many years even without all the other disguises, but we cannot know this. And likely if Faith, Jenny and Elaine were all in a room with him wearing a simple mask he probably would only recognise Elaine if anyone. Because of how well he knows her. But if Elaine were Kumori, all the more reason to hide every recognisable part of herself. You don't argue the impact of Elaine, but we do have the facts that support Elaine over the other two (including that the reveal of Kumori will hurt Harry alot) and yet as I understand your argument to be it couldn't be Elaine because Harry is hyper-observant. But if Kumori's disguise trumps Harry's observational skills then your only reason that it couldn't be Elaine falls apart. Realistically if you were a detective trying to work out who Kumori was would you pick the person who most fit the facts or pick a person who might fit one or two of the facts? Does it really stretch belief that it was Elaine in front of him the whole time in disguise? It is a common trope. Not to mention, it actually happened already with Elaine in Summer Knight. She was with the villains from the start yet he couldn't see it at all. Harry has a huge blind spot when it comes to women, especially in the earlier books and especially ones who seem vulnerable. It is constantly used against him.

Raidem - Ah yes I remember that theory. It would be an excellent betrayal. However as we don't yet know how time travel works in the series, it is still a bit of a long shot. Remember Vadderung says that it is difficult and extremely dangerous to mess with the past - not to mention the Law of Conservation of History (as Harry calls it - the effect Odin describes is that events in the past tend to happen always the same way as there is a quality similar to inertia at work). However I could see it being regular old Maggie Snr who has transcended her Death.
I haven't yet read the Amber Chronicles - it is on my to-do list - but in Amber is the lady that returns a significant character? And does is her return a major plot point? Does it have a long lasting effect on the main character? Was there a lot of foreshadowing? And same applies to the girlfriend. And by "bad actor" do you mean villain or is that a specific type of being or class that exists in the Amber Chronicles?

Thank you Snark Knight, for addressing a few of those points. As SK says, Harry is only observant when it suits the plot. Harry completely failed to notice Molly being in love with him, or that the Grey Lady was Molly (and Leah for that matter), he failed to recognise Lasciel's return in Skin Game and he didn't figure out the Kravos/Nightmare situation until it was way too late - and many more besides. Frequently he only figures these things out at the worst time and only survives by the skin of his teeth. He may eventually figure them out, but I think he has a long way to go. They haven't shown up for a while and I'd say we are well overdue for seeing them so likely another encounter will be in the coming book.

I will do my best to address your points individually:

Groinkick - Very true there is a big difference in that respect, and yes I do feel that Jim wouldn't go into such dark territory normally. However even if that part of my theory is wrong, I believe I am close. Perhaps not twins by blood, but by ritual or some such. I think their connection is a deep one. 

Con - What major plot hole? To your other point, I think it is still reasonable after 10 years roughly that Cowl (if he were Justin) would be interested to see how powerful his former student had become. He also acts rather like a disappointed teacher when Harry proves to be such an unworthy opponent. Justin would be familiar with the Wizard that Harry was at 16, not at 26 (by which point Harry head defeated several dangerous sorcerers, minor demons, started a war with the Red Court and killed one of their nobles, stopped a war between the faerie courts and killed a faerie queen - no minor act considering it hadn't happened for almost 1000 years, and defeated several Denarians). I think he was very interested to see how powerful his former apprentice had become. I had not read that information from the Paranet Papers, which I think is interesting however not evidence enough to exclude Justin or make Simon a more likely Cowl. I was persuaded by the Simon argument for a while but I feel that the theory does not fit the facts as neatly as the Justin Dumorne theory.

Maz - I have answered the first part of your comment above. I am confused somewhat by the fact that you believe that because Harry was a few feet from Kumori he should have recognised her. We know that Cowl and Kumori have gone to great lengths to protect their identities through both physical disguise and magic, why then would Harry the Brawler be able to see through their artifice? They both are likely better at that subtle stuff than Harry is and Justin and Elaine both know how Harry thinks, meaning they could tailor it specifically so that he would personally find it difficult to recognise them. And if Faith or Jenny, why did not recognise them? According to your argument he surely should have recognised them by their voice, smell, magic etc. No, I suspect they are not likely candidates. If you were Jim and you wanted to torture Harry (and by extension his readers) who would you pick to make the most damaging impact? Elaine. He knows Elaine intimately, as you say (though the psychic communication is a spell using neuromancy, he could choose to do it with anyone but it is borderline illegal and he invented it with Elaine and they supposedly trust each other) so the pain of her betrayal will be worse than Faith (who he has met once) or Jenny (who he didn't know anywhere near as well).

Peregrine - A reasonable argument, sure. But being "fairly confident" about something doth buttered no parsnips. Harry is constantly reassessing his history and what he believed. He thought he had no family, he thought his mother and likely his father (malcolm) died of natural causes, he thought Eb was just a nice old wizard who raised him well, he thought Thomas was a terrible monster, he thought Kincaid was human, he thought Leah was out to kill him, he though HWWBH was just some demon (not a major Outsider) etc. What Harry was sure of is only mildly trustworthy at best (especially when it comes to events in his past), not to mention things is only "fairly confident" of. The Name argument is a strong one and my counter is two-fold. One, he wouldn't need to know all of who she is to get her Name right. We are constantly reminded in the earlier books the danger of speaking your Name to another person. You don't need to know exactly what the fish looks like in order to spear it, only to see enough of it and have a good enough method to catch it. A clumsy analogy perhaps, but I am a bit pressed for time. Two, JB unfortunately retcons and creates plot holes all over the shop. I suspect this could be one of them if it came down to it.

morriswalters - SK mostly answered your comment for me. Justin teaches and learned in the school of hard knocks, there are many memories Harry has of his harsh lessons. I suspect he expects everyone to be capable on their own merits. In that particular scenario, one should note Kumori and Elaine are not as major brawlers as other evocation specialists and that the Skavis snuck up on her, which is the bane of every Wizard as JB keeps reiterating. Baby Bickett took me a moment, did you mean the injured Beckett child that Marcone feels responsible for? I mean it is possible but that child wouldn't affect Harry on the personal level someone close to him like Elaine or Susan or Murphy would...it is more a way to get at Marcone if anything. I mean the whole Beckett child thing is not something Harry felt responsible for - he just is sad that it happened.

As for the sword, you must remember how uncommon it is that Wizards fight in direct combat, let alone duels. Harry is definitely an outlier compared to the majority of Wizards and even Wardens. The only reason you see more guns amongst the Wardens is that so many of the old guard were killed and Harry was by far and away leading that pack of new guards carrying modern weapons. Harry having a sword cane that he would use, in any fight, is just plain odd by most standards. Especially as he had it long before he was a Warden. Even only one other Wizard who wasn't a Warden carried a sword, and that was the Corpsetaker. And we really don't know much of his past. Luccio carries a sword not merely as as a secondary combat tool, but as a symbol of office and as an anti-magic tool. Yes it was popular with her generation and with most generations right up until the mid 1900s, but mostly the end of the 1800s. Death by magic (to human mortals) is indeed black magic, but Wardens don't just exist to kill Warlocks. That was mostly Harry's (somewhat justified) prejudices. They do spend a lot of time fighting monsters of the more-than-mortal sort, more so really than they do hunt down Warlocks.

When Mab referred to Death as a spectrum, she was not specifically or even indirectly referring to Necromancy and its ability to bring the dead back to life. I suspect the reason they did not simply raise Bone Tony is that having already tortured and killed him, Grevane arrogantly assumed that he had no more useful information. They picked on Butters as they assumed he might have knowledge that they had missed. Kumori's inability to heal a man who had crossed over does not indicate that could not do so under other circumstances, or that another could not do so using Necromancy. She already had pulled off a feat normally reserved for Gods and other similarly powerful beings. What I believe Mab was referring to was the fact that Mortals believe Death to be the end of existence, which is only true up to a point. By all logic in the Dresden Files, once you die you move on to whatever afterlife awaits you which is another form of existence. True oblivion (where you do not exist at all and are not conscious in anyway) might only exist in certain circumstances, but merely dying in the Mortal plane does not take you there. Perhaps some beings mortals and immortals alike might not have an afterlife (e.g. vampires, gods, demons, faeries and animals) but we have no knowledge of that in the Dresden Files. To answer your last bit about Death Curses - who says he needed to verbalise the Curse? By all accounts only by killing a Wizard before they are aware they are dying (which includes Grevane, who simply refused to recognise he was dying) such as using almost instaneous methods allows you to avoid them. Having been burnt myself, and having witnessed some terrible burning incidents in the line of duty I can tell you that while people freak out, unfortunately you can still think. You are not simply aware of the pain - you are aware of the danger and you are conscious enough to try and take action. It is all the more terrifying because you are aware. Besides, despite how many achievements Harry has achieved killing a highly experienced warden with fire magic because he was scared seems implausible. Justin had survived many engagements and as a Warden would have been very capable of fighting. He may have been caught off guard, but he might have also diverted his remaining strength into surviving. He could have opened a Way or switched bodies - most survivors tend to be the ones that rabbit earlier rather than later. If you wait too long you die. End of.

A few interesting things I would like to point out - Cowl has scarred arms. Scars from what? Burns perhaps? Hell of a feature just to throw in there about Cowl. JB is a lazy writer remember.
Also when Harry talks to his Id - normally there is a secondary message that become relevant later (headaches, lasciel, being too closed off etc). In Dead Beat, he discusses Dumorne with his Id several times. It would fit JB's style to an extent to lay the foundations for Cowl's reveal in the first book he is introduced. 
Finally, I really appreciate the debate from all of you. I enjoy testing the strength of my arguments and hearing others opinions. I would love though if someone can rebut anything or provide an alternative to the points I mentioned better - which some of you have but on balance most of my arguments for their identities remain intact. There has not yet been a strong enough counter-argument or counter-theory that I think rules out my theory as to the identities of Cowl and Kumori. But I am sure someone will have a go and I look forward to the challenge!

Why not? It is messed up, so was that plot point in Star Wars. They never had any kid either - so despite that it is incest it causes no physical trouble. And even though the grew up together from 10 to 16, and even though it might be understandable they had sex (as they were both hormonal teens), it is still weird as even if they wern't blood relatives as they were closer to being brother and sister than just regular friends. I mean they shared the same bedroom, lived in the same house, lived as brother and sister in a way. Being blood related isn't all that stops people having sex (unfortunately history is full of people screwing their relatives, not to mention there are cases of it happening all the time), despite the danger to children born of incest. It is less weird in many ways to start having sex with someone you know a lot less well than a family member. It is very weird grow up with someone in the same house and then start having sex with them, even if you didn't know they were blood you probably still shouldn't be doing it. Knowing that someone is your blood relative should not be the main reason not to sleep with them.

Why Faith Astor? That seems quite out of left field.

Interesting idea. I agree that the is significance in the phrase - hell of a clue bat if an Archangel says that to you. Considering how he uses words in general. Not sure if I totally agree with your theory, but I like the idea that it is related to time travel.

Anyway, good to have you back my man!

While rereading Changes for the millionth time, I was researching Mayan mythology and came across an artifact, rediscovered in Dresden (Germany). It is the oldest surviving book from the Americas dating back to the 13th or 14th Century.  https://en.wikipedia.org/wiki/Dresden_Codex read about it's history here. Of note, it links to a 260 year cycle of the Maya Calendar and the 365 days in a year.  Interesting then that Harry Dresden ends up in the Yucatan with a bunch of expired and/or impersonating Mayan deities.

MY THEORY
The Lords of the Outer Night
I believe that the Red Court is strongly linked to the finale of the series. We know they have links to the Outsiders, and the Lords of the Outer Night (an interesting concept; Outer Night could mean the night beyond the stars, as in the Universe or more likely beyond the Universe - the Netherworld of the Outsiders) surely are directly linked.

The Mayan underworld Xibalba roughly translates to "place of fear" and is ruled by the Mayan Death Gods. Normally it is accessed through a cave, and in some Maya areas the Milky Way is viewed as the road to Xibalba. Xibalba is described as a court below the earth.

There are twelve LoON and the are twelve gods of Xibalba. Two are considered above the others, the strongest is Hun-Came (One Death) and then Vucab-Came (Seven Death). The others are considered demons and rule over aspects of human suffering - causing sickness, starvation, fear, destitution, pain and death. They all work in pairs: Xiquiripat ("Flying Scab") and Cuchumaquic ("Gathered Blood"), who sicken people's blood; Ahalpuh ("Pus Demon") and Ahalgana ("Jaundice Demon"), who cause people's bodies to swell up; Chamiabac ("Bone Staff") and Chamiaholom ("Skull Staff"), who turn dead bodies into skeletons; Ahalmez ("Sweepings Demon") and Ahaltocob ("Stabbing Demon"), who hide in the unswept areas of people's houses and stab them to death; and Xic ("Wing") and Patan ("Packstrap"), who cause people to die coughing up blood while out walking on a road.

So how does Kukulkan fit in? He doesn't quite. Perhaps like the myths, he was a boy born a monster and passed on what he was to those around him. But I don't think so. He is more like Camazotz - the Bat Monster God in form, and more like Gukumatz/Q'uq'umatz in role (as creator god).
BUT, there is another significant group of Mayan gods. The thirteen creator gods who helped create humanity. Not much is known about them other than there names and that they tried to create humanity three times, first from mud and water, then from wood and finally from maize.

It is difficult to reconcile these different groups. JB likely has pieces of information that help link the two - I have a rather grand theory that I will outline in another thread that should help explain that. Possibly he also didn't do a lot of research when naming and designing the Red Court. I find this unlikely, but it remains a possibility given the inconsistencies of the story to real-world mythology. For example, Kukulcan is rarely considered the ruler of the Mayan deities. The most obvious reason that JB chose him is that at Chichen Itza the main temple is El Castillo which is a temple to Kukulkan. This would help place the battle in an exciting location, but makes sense in terms of why the Red Court used the location rather than another.

The Hero Twins
Another interesting connection is the Maya Hero Twins, Hunahpu and Xblanque. They are the oldest Maya myth to be preserved from the text Popol Voh. The Twins are often portrayed as complimentary forces (e.g. life and death, sky and earth, day and night, sun and moon AND INTERESTINGLY male and female). The twins often challenge gods and had a shitty upbringing. They are also talented and use their wits to overcome great foes. They also bring down the Lords of Xibalba. I suspect they are a loose basis for the Starborn in Jim's writing. You might argue that only Harry was there - but I think not. I think Elaine was there, I believe she is one of the members of the Grey Council despite whatever her other allegiances are.

I have more that I might add at another time - but for now I will say that while I acknowledge this theory is incomplete and loosely fits the "facts", I believe that Jim has certainly done his homework here.

and where is #8 proven at in canon... or anywhere else? Never have a seen that even hinted at that I recall..

Con is correct - see the Even Hand story. But is makes sense too; they like to mishmash body parts.

Indeed, specifically Outsiders are in-text referred to as coming from the the Nether World. There is definitely a big link. Jim will of course have his own spin...but even in Hellboy there are links between, and in Warcraft, and D&D between the Celtic/Norse/Greek/Faerie/Elves and Demon. And JB uses all those, plus regular mythology and a few other fictional works.

It is just we know so little about them in-world.

Fact 1: They were related to the Fae and the Jotuns. The married, fought and were eventually driven back to the sea.
Fact 2: The currently are a nation of outcasts, not just the original Formor, but dark gods and spirits united together.
Fact 3: The Cantrev Lords look like frogs and have powerful, water based magics. (This further shows Harry's understanding of Magic is seriously limited).
Fact 4: Their servitors (turtlenecks) steal magical talents for an unknown purpose. They use both modern and ancient and arcane technology. They are sort of like magical cyborgs (but instead of machine parts, they have weird animal and monster parts).
Fact 5: The Red Court was somewhat keeping them at bay, or at least less active. When the Red Court fell, the Formor quickly and fiercely took its place.
Fact 6: One of their Cantrev Lords is Lord Omogh (who I can find not much info on), and another is Mags who was defeated by Marcone.
Fact 7: Their leader appears to be King Corb, who's court is located in Lake Michigan (I mean come on JB, change it up a little, we get you like Chicago but not everything of significance has to be there, there is an entire PLANET outside of America, let alone Chicago).
Fact 8: They created the beasts used by the Denarians, the hybrid monsters. This suggests dealings between the two groups.
Fact 9: The have had dealings with the Corpsetaker...and Evil Bob. This suggests a link to the Disciples of Kemmler and perhaps even the Black Council/Circle.

I also believe Harry is weaker than the Ladies, and Mab vs Odin seems tilted against Odin (nevermind the Erlking vs Mab, which is even more lopsided). And the Gatekeeper is a very old Wizard.

Very true but my point is that he doesn't likely outrank Odin in power, (who himself is several millennia old.

Sibelis, it has always seemed the Gatekeeper is more dangerous not because he has tons of magical might, but because he (like a true JB wizard) has knowledge and ability to affect enemies most vulnerable elements. Which is not to say he isn't a bad ass wizard, but if Harry or Eb is like a hammer, Rashid is like a scalpel.

We're limited in what we know about the Gatekeeper.  But JB has teased that he might have thrown down with Demonreach and by inference the last warden.  Drag, thump.

Sorry Morris, believe that one has been debunked by the man JB himself. He said what caused the limp was the last friggin ice age, which one could interpret as the island being physically limited (like being cut off from the ocean perhaps). Check out Serack's WOJ page under demonreach - pretty sure it has the direct quote.