There are really only three main characters in Boku no Pierce Girl.

Yuumi Yuumi is the girl twin. She's spunky, confident, opinionated, and a tomboy - she's a better athlete than her brother, able to hit home runs in baseball with nonchalant ease. She's also very impertinent and tends to rush headlong into things - from running off to play ball in her brother's place to making impulsive bets with Kageura. She loves her brother Yuuki deeply - they seldom argue and do everything together. When she puts on his baseball cap, she looks very much like him - except for the earring sparkling in her earlobe.

Yuuki Yuuki is the male twin, and he loves Yuumi with the same deep affection that she has for him. The two of them are very close, sleeping in the same bed like small children even though they're in eighth grade. Yuuki is calmer and more serious than his sister, and wears glasses at times to study. In general, he seems much more responsible and thoughtful than Yuumi, but he lets her override him. On the other hand, he is very like his sister in that he is quick to react to Kageura's taunts.

Kageura Kageura is the dark-haired star pitcher of the Koto-ku Shadows, and is quite surprised when the earring-wearing player of the Minato-ku Nighters hits a home run off one of his pitches. He quickly figures out that Yuumi is a girl - and that he would very much like to go out with her. However, his pursuit of her brings on more complications than he could have predicted - for his actions strike friction between the close-knit twins, who have never willingly let anyone come between them before.