It looks cool, but two things - one large and one small.

The small: About the IoP - In OW all Denarians have a note saying that while the powers come from a coin, the coin is only separable from the wearer by death or conscious choice, thus it doesn't have any of the drawbacks that's the source of the rebate from IoP, so no point in making it one.

Gotcha. I don't like OW very much so I don't pay much attention to it. Too many skillsets that don't make sense or custom powers that are severely unbalanced.

The larger: I think the premise of "DCP is based on the Denarians" is incorrect. None of the Denarians are statted with Demonic Co-Pilot. What DCP is based on is actually the Hexenwolf. It's even stated pretty straight up in the sidebar next to the power, and it's specifically mentioned under Hexenwolf in OW (p93). The only place it's in use in OW is for Agent Benn, the other Hexenwolves refer back to her statblock.

My bad. Again, since I don't pay much attention to OW I missed the Hexenwulfen bit.

That said, I really like the concept you have going here! I'm really not experienced enough to say anything about if it's balanced or not, but the concept is cool, and could really explain someone like Ursiel. I like how you tied the various parts of it directly to Harry's experiences.

Yeah, well, I had the Denarians in mind through the whole thing. Maybe I should rename it to something else to signify that it's not the Hexenwulfen thing.

I like it as well:

I think you need to be more specific with The First Taste is Free

It seems very vague to me.  How much refresh worth of powers can I access for free?  At what point do they get withdrawn? etc...

I've left that intentionally vague because I want to leave it up to the GM. Harry gained Sponsored Magic as his free taste, which is about as high a refresh cost as any of the powers regular PCs get (Mythic Whatever and the likes is something I'd keep for NPCS only).

I don't like the consequences attached to the number of scenes.  Just have any consequences invoked.  It just gives the spirit that much more leverage on the character in its attempt to take him out.

Yeah, I'm not so sure on that myself either. I think I'll edit to make it an all or nothing thing. Lash never possessed Harry outright, after all, and Nicodemus' order to paralyse him could easily be explained by an attempted Compel on Sponsor Debt.

The only things I really consider necessary to represent Demonic Co-pilot are access to sponsor debt (and the resulting Compels), and an appropriate character aspect.  That's it.  Nothing else.

I cost this at [N/A], meaning that it's not even worth losing a Pure Mortal bonus, if the character retains that.

I have it as more than that because the spirit acts as an advisor as well as a method of gaining powers. Hence the Superb workspace.

So during my game one of my players suggested putting Demonic Co-Pilot on an NPC because it would fit the character concept well. I'd never really looked at the ability before, but now that I have I feel it could be so much more than it is. DCP is based on the Denarians, which I feel encompasses a whole lot more than just transformation, so I've added a few things to make this more accurate. I've also had a look at the custom powers thread but I don't think that version works particularly well. So here is my attempt.

Demonic Co-Pilot [-VARIES]
Description: You have a spiritual parasite living in your mind, bringing its baggage along with it. This may come with a variety of benefits, but always comes at a cost.
Musts: You must have an Aspect reflecting this Power. You must take at least one ability linked to this Power. You may not take this power more than once.
Skills Affected: Many.
Effects:
Arcane Advisor: The spirit living in your mind has a lifespan measured in centuries rather than decades, and has gathered a wealth of knowledge and experience over the course its lifetime. This allows it to act as a workspace (as detailed on YS140). You may ask the entity questions up to Superb rating for the cost of 1 point of Sponsor Debt.
Sponsored Powers: On top of the knowledge the spirit possesses, it may also choose to grant you access to a variety of powers within its purview. Choose which powers you wish to take from the Creature Features, Minor Abilities, Nevernever Powers, Psychic Abilities, some Shapeshifting abilities, Speed, Strength, Recovery, and Toughness categories. Sponsored Magic is also available, though regular spellcasting powers are not.
Sponsor Debt: Many aspects of this power work hand in hand with the Sponsor Debt mechanic (YS289). If Sponsored Magic is taken as a Power, the Sponsor Debt mechanic is merged with this Power.
The First Taste is Free: Some spirits may be willing to give access to a single power without attached Sponsor Debt as a way of gaining the person's trust and/or reliance. As seen in the DEAD BEAT casefile, use of this 'free' power may come with other consequences such as allowing communication between spirit and host.
The Price of Power: Every time you use a power linked to Demonic Co-Pilot, the spirit accrues 1 point of Sponsor Debt, which may then be exchanged for compels on a 1:1 basis according to the spirit's agenda. The spirit is also able to deny use of any power, should it wish to.
The Struggle for Control: If the spirit so chooses, it may exchange any Sponsor Debt it has stored for a mental attack (defend with Discipline) with a strength equal to the number of Debt Points exchanged. If the character is Taken Out by this attack, he becomes permanently possessed by the spirit.

I've designed this power to be a double edged sword in a big way. It has access to the majority of powers, though I would leave it up to the GM's decision as to which powers a given spirit would be able to impart. Arcane Advisor is the greater of the two benefits it has, in my opinion, since it is essentially a portable superb workspace, which makes for a very broad spectrum of possible declarations.

I wanted to work in Item of Power somewhere as a way to tie this in to the Denarians more, but then I figured it would work the opposite way around. Take Item of Power with Demonic Co-Pilot as one of the imparted abilities.

The First Taste is Free is meant to portray the slippery slope. Get the victim used to your power, so you can deny it later or lead it further down the slippery slope. In Harry's case this would have been portrayed with hellfire, which Lash then used via compels to make Harry angrier, which could then be used to later make him break a Law. Slippery slope.

The Price of Power and the Struggle for Control are two sides of the same coin, really. I like the idea of the spirit being able to decide between short term goals, such as throwing the character into situations where they may need to buy more powers (or temporary powers), or long term ones that would allow it possession over the character. This would be especially good for characters that are used over multiple campaigns or over one long campaign as the spirit would be able to acquire a lot of Sponsor Debt over that time. Could lead to some interesting situations where the PC is unwilling to rely on the powers he's grown accustomed to using, while frantically searching for a way to get rid of the spirit in some way.

So what do you guys think?

Couple of points:

Firstly, you can kill with a gun. This doesn't mean spells that kill are Kosher. For that same reason, using time magic to look back in time isn't kosher just because psychics can do it, or there are other ways to do it. Psychic powers are not the same as magic, no matter how much it may seem like it.
Secondly, like all magic, IT DEPENDS ON THE METHOD.

Basicly, it all boils down to the method. Look at the casters preferred methods for doing things. Are they being indirect about it? Are they physically going back in time? Are they temporarily giving themselves psychic powers? It all depends on the proposed method.

Well, that's a matter of perspective. I doubt it would matter much to a being that exists at multiple points in a given timeline (as some oracles are supposed to), but the White Council doesn't differentiate much when it comes to things like this. Harry's view of the Alphas (that they're all just doing a very specific brand of magic) is almost certainly the same as the White Council's view. So if someone broke the Sixth Law with a 'psychic power' I can't see the Council approving of it any more than if a Wizard did it.

Given that psychometry doesn't break the Sixth Law in the RPG, I'd say getting information from the past is fine no matter what method you use. Sending messages back (as long as they're subtle, as Bob says in Proven Guilty) would be toeing the line. Affecting the past in a grandfather's paradox-esque manner would be breaking the Law with universal consequences.

There have been a number of options listed, but I like the idea of lowering the Fetch's toughness by one notch for the next fight, given that its catch has been satisfied by the iron trap. Also, I'm pretty sure Faeries can't sense iron (Cold Days spoiler).

I would think that the Wardens would not be particularly fond of slavery, on the whole, since it's not a long step from owning slaves to making them with black magic. That said, they wouldn't have enforced it since it was a matter for mortal politics.

I see the old Wardens as being much more intolerant of Lawbreakers than the newer Wardens, mostly because of the mortal cultures they grew up in. Morgan was raised in 1800s Brtain, at the height of the Empire and East India Company. His wasn't an era where prevention was emphasised in the justice system, which shows pretty heavily in his views on Black Magic. Ramirez grew up in 1990s California. That's a pretty huge difference in culture right there.

Cheers for clearing that up for me. I think I've got a better idea of how the next fight with the FBI will go now.

I wouldn't expect trouble if he uses it for Powers.

Yeah, that doesn't bother me so much because you have to pay full refresh cost for each power. Plus it keeps the game interesting.

What worries me is the possibility that he could pick up a big skill from a powerful NPC and then keep it forever. One lucky opportunity lets you be overpowered indefinitely.

Mimicking the Merlin's Discipline makes using Refinement to boost your control seem weak. Mimicking Shiro's Weapons is way better than True Aim. And so on.

I've solved that problem by forcing him to keep to the skill cap. He can only take skills up to Superb, with higher ones being lowered to Superb when he takes them. It's mostly for balance issues (as you said), but I've fluffed it as his body's way of minimising the dangers associated with stealing powers. I'm just waiting for the day he tries to take Sponsored Magic from something.