Expanding on this idea slightly, instead of being cursed to transform when in water, running water is gradually washing away the spell that keeps her transformed. By the time the game takes place the spell is nearing its end and he/she's transforming into a mermaid more and more often, often at completely random and inopportune times.

Im liking maybe a combination of ideas, where they upset a Sidhe and was cursed and, by the time the game starts, and the curse becomes more prominent where its starting to wear off at inopportune times.  I really do like that cause it does give the character drama and interaction with the Fae, which is something I do want, and I like what someone said about the learning to be female moments with male mind as I can see some random comedy from that.

My questions to this though would be: what type of inopportune times? Like near water? Just randomly? Could I get some examples?

Also, what template should I use? Is changeling the best one or should I go for a different one? Thats kind of my hold up.

I just meant that compared to giant octopi sightings, the presence of a mermaid in Oklahoma is less weird.

Oh, gotcha gotcha. As for whats been suggested and such, do you have any input for the character creation or what would be fun or creating trouble? Any suggestion is welcome as Im looking to have a solid character idea by the time the first session happens.

Poor guy. With the Fae I could actually see that happening.

Trouble Aspect: The Little Mermaid (But Actually I'm a Guy!)
Compel for aquatic creatures to annoyingly burst into song at him or have Ranma 1/2 style mishaps occur - like your best friend having a crush on the "other" you. I could see Maeve doing this to somebody.

Power List (stealing from Haru slightly):
Glamor [-2]
Human Form (Involuntary, touching water) [+2]
affecting:
 Aquatic [-1]
 Inhuman Speed (representing the fin) [-2]
 If you wanted a water theme you could potentially add in Channeling: Water (-2), although using Incite Effect (which  can be found on the Resources page and has a ton of upgrades) would probably work better. It could also be used for  siren-type effects.
 Sex Appeal (?) - maybe whatever Fae did it would at least (un)luckily make him shift into an cutesy little mermaid surrounded by adoring sea critters? Like Aquaman but with a severe hoarding problem.

Edit:Originally I was just going to post for fun but now I'm going to try to offer some real help. I think one thing that might help you to decide is determine what conflicts down the road do you want to see with the character? Players have a lot more sway over the narrative than is typically the case in other games. If it's a female and she's cursed, what's the disadvantage of being cursed or why is she cursed? Does having water tossed on her cause her to grow gills and make her unable to breathe air? Was her mother (or father) a merperson who serves the Court of Atlantis or another group of Fae so the character has to do something as a favor to them? Did she just so happen to pick up a cursed conch that made its way inland and now every so often she gets mermaid impulses to drown somebody (similar to the Loup-Garou and the full moon)?

For the male to female trickster'd PC is he cursed to be a woman all the time now or is he a guy that just turns into a female mermaid when exposed to water? Maybe in exchange for being made a really good swimmer by a Fae he offered to be the Fae's servant - not realizing that all of the servants were female (whether they started that way or not - kind of like Lea's dogs). Maybe he was just a star swimmer who accidentally scorned a water nymph and she wanted to punish him for it (could tie into the broken promise idea you had)? He could either spend the game trying to find a way to break the curse or just have complications come up in his life that result from being an (unintentional) Little Mermaid.

Perhaps thinking into some of the characters' other Aspects would also help you to decide which choice would make for a more well-rounded and engaging RP experience. Do any of the other players at your table have an idea for what they want to do?

Im still very new as a player and GMing, so most of our players are still trying to figure out what they want to do and build. So, NOT yet but the ideas are flowing. We're all trying to be creative so that its not a typical roleplay and trying to stay within a balance of the power. Like, no one wants to start off flinging power like Dresden or Michael. We want lots of room for character development. I can provide those details as they emerge.

One question before I go on, for the Fae, as they would do something like that, why would Maeve do that? Like, do you think there's a compelling interest for her to do it outside of just because, even though Fae fit that bill?

As for the curse, I'm FAVORING kind of a mixture between a Loup Garou where she gets urges to be a mermaid, in addition to the normal situations where she would have to defend herself.  I was thinking for that drowning as a possible impulse, some inspiration from normal mermaids and Pirates 4.

In addition, me personally (as its my character haha) I'm favoring a male turned female all the time, with the addition of the curse turning her into a mermaid. I was thinking it would be amusing of a 'learning yourself again' and getting into those awkward situations as simple as wearing a dress as a distraction but acting all manly cause of no experience as a lady.  That's where Im a tad held up as I do want the Fae involved but I dont know how. I do like the idea of the Winter Court being involved however with Maeve or maybe even Mab, as shes the one who would seem powerful enough to permanent gender bend an individual and change their species at will.  But, I dont know how or what history.  That's also where I was thinking that the drama could be as its living with the consequences of being a mermaid as, well, I like the idea of being tempted but I also like it as well of no temptation and 'it is what it is and youre stuck with it period.'

Also, thanks for helping me out. I appreciate it. Im really doing my best for trying to nail my character down and learn it so I apologize for all the questions and newbie ideas and such.

Ill also be on for a long time so Im available for responses and such tonight

Good afternoon everyone! I hope everyone's day is going well.  I just recently purchased the Dresden Files RPG books Your World and Our World and Im trying to build a campaign with some friends of mine for an Oklahoma City campaign.

With that said, I am wanting to have a mermaid as one of the main characters but make her more 'cursed.' I figured a changeling would fit that roll as it could go either way of a female accepting's a Fae's temptation which backfires or a male turned female mermaid due to the natures of the Fae and their whimsical looks on humanity and toying with them, either backstory struggling to try to return to humanity but tempted by the power of the Fae.

Regardless of either one (and I would like to hear your opinion on either and which would best suit the backstory,) I am having difficulties building a mermaid as a PC as I am reading as, honestly, Im very new to this and Im not even sure if they would fit as a changeling. Could anyone offer some advice and tips on what would make a mermaid in the Dresdenverse as there isnt really any input or examples that one could use. I would sincerely appreciate it and it would help me with helping others build their characters as well. Thank you so much!