Tsukino Akari

A third year student of Seiai Academy, the former student council president and current girlfriend of Ryouji's older brother, Kazumi. She is very "together", and acts as a kind older sister to Ryouji and his cousins. She's been going out with Kazumi for awhile

Spoiler

and is quite experienced in bed, calling him "master" and engaging in all sorts of deviant play.

Since her parents left on a trip she's been staying over at Ryouji's house.