Koyuki Sasamori

Because of her Iris that allows her to see people "qualified" (O above the person's head) or "unqualified"(X above the head) to any task she thinks of at the moment, she is chosen to find a new Student Council President. After it becomes more difficult than she thought, she turns to Mizushima Toru, a person "qualified" to help her. Because of her popularity in school, he's far from being delighted with her, but in the end he helps her...

Spoiler, click to view

It turns out the person qualified to become a new Student Council President is the only person she couldn't have looked at - herself.

Very grateful, she tries to become closer to Toru, even though he's clearly avoiding her.

Spoiler, click to view

She eventually falls in love with him... Which was pretty obvious from the start, but still - a spoiler it is.

Koyuki is cute, cheerful, helpful and popular. She's in class 2-6 along with her best friend Yuki Asahi and Toru's best (and only actually) friend, Shinozuka Hijiri. Koyuki can't say 'no' when someone asks her for favour, so sometimes she's quite busy. Also she's always very concerned about others which is sometimes seen (by Toru, for example) as just her being meddlesome.