She is one of Kei's close friends who also pursues a romantic relationship with Kei, but towards the end of the series discovers her affection for one of her teachers, Yamada. This inappropriate relationship is only ever hinted at, but it's more than obvious that something is going on between them when Yamada is confronted by the principal about him seeing Yamada and a girl from their school walking together. Her parents own a grocery store and she often has to go to deliver food that people ordered.

Koishi has a happy-disposition to her personality and is only ever seen crying when it comes to matters that involve Kei. One of the things Ichigo would get irritated at was that Koishi would never act on her urges and in effect she waits too long, and by the time she does tell Kei her true feelings, he's already hopelessly in love with Mizuho.