Misasagi Mizuki♂

The first friend Ryou made when he enrolled in the new school. When they first met, Ryou assumed Mizuki was a girl. Mizuki very perceptive and understanding of a maiden's heart so Mizuki gives Ryou a lot of advice about girls while also promoting a relationship between them of more than friends. Mizuki's clan is know as the one who controls the city and is very strict, causing a lot of pressure to fall on Mizuki, resulting on Mizuki having ocasional violent anger outbursts.

(spoiler hidden)

Future: In the prologue's future, Mizuki isn't directly mentioned, but Mayuri says there were rumors about someone from that school that lost control in anger and killed a servant and was arrested...