There is a strange individual around town. No one really knows the truth of anything about this person. True name, age, gender, past. All of it is a mystery, none of which the individual is inclined to reveal. It may not even be human, but for the most part no one really can say anything about it for sure.
This individual is a shapeshifter. That is about the only thing anyone can say for sure. People have seen the person shift their form. Sometimes male, sometimes female, their favorite hobby is to play with the desires and taboos of those around them. Sometimes s/he fulfills personal fantasies, sometimes s/he intentionally picks a form to make the other person uncomfortable.
Most likely s/he has a website/email address where people can make requests and sometimes s/he goes looking for people to have fun with, at least sometimes the person may not even realize it is the shapeshifter, sometimes they know, and sometimes they suspect but can't/won't confirm. The shapeshifter is known to occasionally attend classes at local schools as well, but it is unknown if that is a confirmation of true age, or if the shapeshifter has influence with people in power to allow it (s/he does have such influence, but it may or may not be necessary for this particular arrangement).
This can a lot of ways, along with one or more characters, the shapeshifter could take forms of:
Random male with female partner
Random female with male partner
Random female with female partner
Random futa with female partner
Form of the other character's friend/crush/lover/relative/lover of someone they know.
Form of a fictional character
Form of a teen for older character
Form of older character for a teen
Same aged form
Possibly other things too.
The other character could be a stranger, a classmate, maybe even a casual friend (in that they do stuff together, but the friend still doesn't really know much about the shapeshifter). More likely than not the truth of the shapeshifter will remain hidden. Don't have much of a story planned at this point, it will largely depend on the other character(s). It potentially could include things like cheating/affairs or 'fake incest' (in the sense of taking the form of someone else's relative), possibly sometimes doing it openly, sometimes pretending to be the one s/he is impersonating. Possibly with the idea of tempting the other character to do something they might not otherwise.