I had a couple classmates that pretended to be vampires back in elementary and middle school. They’d pretend their Koolaid was blood, complain about the sunlight, and bite their friends a lot. Not enough to draw blood, though. I haven’t kept up with most of them, but one guy is a teacher now. He seems pretty normal.
I used to pretend I was bionic. Convinced the other kids that the visible blue veins in my wrist were wires, and would go above and beyond to exert myself so as to always appear stronger and faster than the other kids. I don’t remember how long it went on, but it was for a while. I was like 7 or 8 maybe. I’m not sure how much I actually “believed” it, I think I was probably more like a method actor that took things way too seriously for a while, :-p