A member of the Seto Group. Masa is the one who stole Nagasumi's first kiss (he was actually performing CPR, but anyone would use that as an excuse to say they got a kiss from a man as beautiful as him). Nagasumi and his mother are very attracted to Masa while father is very jealous of him. He also transfers to Nagasumi's school, becoming his math teacher.
Before Masa lost his memory he was Shiranui Akeno's brother.