When Peter was six years old, he picked his nose, and his fingernail was a little too sharp, and he had a nosebleed. This was hardly unheard of. The nurse told him to pinch his nose, hard, for ten minutes, and it would stop – and if it didn't, pinch it again.
It didn't stop. He pinched it for ten minutes, then another ten, and another. He kept pinching his nose, in ten-minute intervals, checking again each time, until his teacher noticed him doing it two hours later and told him to stop that, and the blood started flowing again. Then he went to the hospital.
There was no history of hemophilia in the Pevensie family. That didn't stop him having it, it just meant it came as more of a shock. Peter wasn't allowed to play football anymore, because he might get a bruise on his brain that could kill him. He certainly wasn't allowed to play rugby, where that was even more likely. He was allowed to swim, which he liked; he was allowed to lift weights and do calisthenics, which he found boring; and his mother told him tearfully that he must absolutely never pick his nose again.
So he swam, but not competitively, because if he had a really bad joint or muscle bleed it might take days to recover and he'd have trouble training around the interruptions. He stopped playing football, and didn't really have anything else to talk about with the boys he'd played with, and so he wasn't really friends with them anymore. (He could have watched football just fine, but it didn't feel right.) He studied diligently, and he played computer games with other boys who didn't like sport, and he never picked his nose again.
The Pevensies moved to America when he was fourteen. (And Susan was thirteen, and Edmund twelve, and Lucy ten.) Lakeview, a little town with a handful of rich old families you didn't want to cross, which his father said was just fine with him because he'd never crossed anyone in his life, whether they were probably gangsters or not. They mixed in decently; Peter made a handful of friends, Su made rather more, Ed made somewhere in between, and Lu didn't give a damn, which attracted its own crowd. He was proud of them all. And some of his new friends could play Dungeons and Dragons, which was really quite fun.
He's settled into a firm equilibrium, by his senior year. He's exempt from phys ed, instead swimming every afternoon at the local YMCA. He has his D&D group, and his family, and mostly everyone else ignores him, which is fine. He's applied to a handful of colleges, and some have expressed interest back at him. No one bothers to bully him, which is welcome. He's got a car.
He is, in fact, driving that car to school, with Susan and Edmund in the back. It started raining a few minutes ago, abruptly and intensely; they'll be lucky to get from the parking lot into the school building without getting soaked.