Koizumi is a young girl with blue hair and red eyes. Her looks and figure has the guys in school turning their heads, including Saji. However, Koizumi expresses her rejections from the proposals of the boys by physically assaulting them. Out of all the guys she meets, Saji is her favorite target to take her frustrations out on. In the beginning of the series rumors go around saying that she is a lesbian and has sex with older men for money. But soon after that is found out to be untrue as far as her being a prostitute is concerned. She still hates men and apparently had a former lesbian lover. (hints suggest that she started those rumors herself to keep people from getting too close to her) As time goes on, in her interactions with Saji she eventually starts to open up. Although she doesn't hold back from attacking Saji when he does something perverted she will often state that she acts differently when he's around. At some points she will get jealous of Saji doing perverted things to other girls which promptly leads to a sound beating from her or she will often often shout out her inner feelings revealing her growing fondness of Saji. It should also be noted that despite the fact that everyone thinks that her and Saji are a couple she does nothing to dispel these rumors and doesn't really seem to care. Deep down she really cares for Saji and credits him along with her friends Ryoko and Maki to being the reason she decided to stay in Japan.