Yui Kotegawa (古手川 唯)

Yui is a girl in Rito's class who seems a good deal more conservative than the others (except in the anime, where she's downright being portrayed as a school nazi, ruling the hallways with an iron fist), which is partially caused by her prettyboy brother and his habit of going out with different girls on a regular basis. She quickly comes at odds with Lala and her free-spirited personality, which she sees as a hindrance towards making the school a more peaceful and better place.

Eventually, she loosens up a bit, but she's generally more on her guard than the other girls. She becomes a part of Rito's harem all the same.

It is also implied (albeit rather briefly) that she might have feelings for Rito.