okay that's a theory and what imo could explain the thing but
otherkins are people that feels like they aren't human, they feel a disconnection between their mind and body. they most relate to an animal or fantasy creature, and feels like their bodies isn't the right body.
i think fictionkin might be the same thing. some people relate to one or many characters so much that they feel like they ARE the charater, they kind of forget who they are and loses themselves to one character. They become like a fictionkin, thinking they're not in the right place. I think those kind of events happened to some actors before, they acted a character and gave themselves so much to the role they lost themselves to the character.
And then, there's the trend thing. It's really scary how frequent you see the fictionkin thing. well i think it's because people might not understand exactly what "real fictionkin" are? but like, a lot of people can relate to many characters right? haven't you ever read a book/watched movie and completely understood what a character was going through? and relate to how the character acts as an answer to their problems?? ect?? to be honest i did a lot of time, i thought "this character is like me". and i believe a lot of people knows what i'm talking about? well i think the majority just works like that.
and then there's the real real trend thing where people will just turn fictionkin just for the sake of "mixing" with the others on tumblr.
But then i am not otherkin or fictionkin, and i know i'm a pretty rational person trying to give a logical explanation to everything.
so yeah that's the explanation i'd give to all the events going around but i guess if you want to understand the whole concept in the eyes of a fictionkin then u should read a fictionkin's readmores maybe haha