Sawaki Rinna♀

A second-year transfer student. Wataru meets her at Takashi's bar while he's drunk. They chat and then leave. The next morning, Wataru awakes to find her asleep in his room wearing only bra and panties. She wakes, he tells her he can't remember a thing from the night before, she slaps him and leaps from his second-story window.
She officially enters his school, joining his class and dorm the next day. She acts sullen and obstinate to both students and faculty. She skips the welcome party that the other dorm residents arrange for her, telling them that she doesn't want to make friends and faces the pain of parting with them in only a years' time. Wataru shouts to her his vow to that he will force her to become friends with them.
Takashi informs Wataru that he and Rinna first met in his bar. Realizing that Rinna's general aloofness emanates from his having forgotten about her, he challenges her to compete with him in the upcoming school marathon to settle the matter. If he wins, she will participate in dorm meals, and if she wins, he will leave the dorm.
Wataru arrives at the marathon exhausted due to a sabotage, but takes a shortcut during the run and arrives first at the finish line. Rinna welcomes the excuse to join the group and joins them in another welcome party that reboots her introduction.

[Edited from Wikipedia]