Kamioka Yukari

Yukari is one of the first year students at the school who lives in the same dormitory building as Mizuho and Mariya, who also is her onee-sama; it is a tradition for the underclassmen to serve their respective senpai any way they can. Much like Kana and Mariya, she is a very energetic girl, although unlike them, she is deathly afraid of ghost stories and anything scary - a fact which Mariya has often used for her own amusement and which initially caused some trouble when Yukari met Ichiko.

Yukari is on the track and field team along with Mariya, though her main reason for joining was due to her older sister-in-law who had urged her to come to this school and who was also in track and field. However, Yukari soon comes to discover that she really does have a liking for track and field and continues activities in the club even after a time of losing much of her self-confidence.
Her another name is "Kohaku no Kimi" at Seio Girls' Academy.

Spoiler

She became the student council president and the captain of the track and field club.
Incidentally, she retires from the club just before Mizuho's graduation and she begins to study in earnest so that she will enter Mizuho's university in her path.