Setsuna Kiyoura

Also in Year 1, Class 3, Setsuna is a member of the class committee and is childhood friends with Sekai. Their mothers are good friends and they work together at the same company. Setsuna is constantly worried about Sekai and tries to make sure that Makoto will keep Sekai happy. This leads her to go so far as to try to eradicate Kotonoha from Makoto's love life by cutting short any interactions they had while she was around, although this was only done for Sekai's sake and she didn't harbor real ill feelings towards Kotonoha herself. Although silent-looking and almost emotionless, she is conscious about her underdeveloped body, especially in comparison with Kotonoha and Sekai.

(Source: Wikipedia)