An uptight, high-strung girl who is in Rito's class during the second year, Yui has little to no tolerance for anything she does not consider to be "acceptable behavior", ironically despite this seems to secretly desire for Rito to do them to her (where in the OVA 5 she states "you can only do naughty things to me"). Her haughty opinions, quick temper, and habit of speaking her mind often results in her vocally reprimanding classmates. Yui quickly grows annoyed and disgusted by eccentricities and bizarre actions, particularly with regards to Rito and Lala.
Unfortunately, almost every encounter with Rito usually ends up with her embarrassed, naked or groped, and although though her nature prevents her from easing her behavior, she does slowly warm up to his usual antics. After Rito saves Yui from a group of delinquents, she develops feelings for Rito. Though her inexperience with boys leaves her unable to understand her feelings and continually denies her feelings to herself.
When Rito usually asks what's wrong, she says the exact opposite of what she wants to say and then hurts him in some way (a typical Tsundere). It was only resently that she has realised that she is in love with Rito, a fact she is annoyed about given her opinion of him. She did however almost try to have sex with him after getting caught in the rain together (on the condition that he only ever sleep with her) and has been seen fantisizing about Rito asking her on a date.
A common gag is that newer characters cannot remember Kotegawa's family name, presumably due to the fact she is generally an unwilling participant in Lala's frequent schemes. Also she really likes cats – owning cat based books, having a stuffed cat, decorating her room with cat shaped things, shaping her chocolate into a cat's face, and even having a cat-themed floaty, something Rito seems to find cute about her. According to Mio Sawada, her measurements are B88, W59, H87