A popular girl at Nekomi Tech, to the point where she considers herself the "queen" of the school. She's also quite rich, coming from a wealthy family. She has her own apartment, an expensive car, and gets to throw lavish parties with very important people. When Belldandy appears, all the guys' attention turn to her rather than Sayoko. Mishima begins to get jealous of Belldandy and wants to make her upset by seducing Keiichi, even though she does not have any interest in him. She even goes to the lengths of teaming up with Toshiyuki Aoshima and Marller to bring her down.

Spoiler

Gradually, as the series progresses, she becomes less and less antagonistic, even offering Keiichi relationship advice and foiling Marller's schemes. It seems as though later she develops feelings for Keiichi.

(Source: Wikipedia)