A girl in Haruo's class throughout junior high. Goes to an all-girl school with Onizuka throughout high school. Previously an introverted girl who spent much of her time studying alone, she develops into a more sociable person and develops an interest in gaming after spending time with Haruo over a new Neo Geo MVS that gets installed in front of her family's store.

Spoiler

She has a natural instinct for fighting games, which transforms her into a powerful forced to be reckoned with as both a gamer, and a rival for Haruo's affections. After a fateful showdown at her and Haruo's usual stomping grounds, she developed a bit of a sadistic side whenever Haruo's around. This frightening skill garnered the attention of the "Mizonokuchi Force" leader, Nikotama, who helped Koharu truly awaken her love for fighting games.