Naoto Kadomatsu

A third-year student at Jousei High School and a member of its boxing club. He lives in the same apartment building as Minatsu but attends a different school, in a different grade. He is a high school boxing champion.

[Spoiler] He falls in love at first sight with Minatsu's alter ego Maria, and becomes Shuu's rival in boxing and in trying to win Minatsu's affections after learning that Maria and Minatsu are the same person. [/Spoiler]

(Source: Wikipedia)