Kiyoura Setsuna♀

Setsuna Kiyoura is a student in year one of class three, who like Kotonoha, is on the student council as a class representative. One of the more inscrutable characters, Setsuna is a keen, focused and habitually tacit girl who tends to have and keep the most level head of her peers when situations go awry. She is Sekai's best friend, having grown up with her, and to a considerable degree is overly solicitous of her, to the point of hurting others to ensure that Sekai is happy. In Summer Days, Setsuna is shy and timid, in contrast to her more serious portrayal in School Days. Her mother, Mai, only makes an appearance in Summer Days. With the exception of Kokoro Katsura, Setsuna is the shortest character of the main cast, and is often mistaken for a junior high school student, a trait she is well aware and conscious of.

[From Wikipedia]