The big mystery is what is fives name but I don’t think he has one. At least, Gerard way and Gabriel Bá never intended him to have one. I know in the show he calls out for his siblings by their names but in the comic book when he runs away into the future they still didn’t have names. He found a library and saw Vanynas book but he only knew her as number seven which means he didn’t know the name Vanya which means they didn’t have names yet which means they probably chose their names themselves when they left the Academy.