Koharu is a third-year from the Astronomy club. She is extremely tiny, resembling a small child.

Spoiler

She's a stalker who engages in constant voyeurism and is in love with Keiki's lolicon friend Shoma, but worries that despite her "loli" appearance he'd still refuse to date her on account of her being older than him. She thus uses secretly-taken photos to try and blackmail Keiki into helping her.

(Source: Wikipedia)