天川 ちなつ

Chinatsu  is a second-year student at the private Tōrin high school and is a classmate of Naoki's. In addition, she's Naoki's cousin ans childhhood friend. As Chinatsu is concerned about Naoki at all times, she commutes to school with him and is always around him during school hours. She appears to be an airhead, and a self-proclaimed bride of the hero. 

This "bride" term is not entirely made up by her, in fact, Chinatsu was assigned to be Naoki's fiance from birth, but after Naoki's qualification of a heir to the Serizawa family has been deprived, their official unity has been broken. So Chinatsu has become the main heir of the Serizawa household.

 and translated from http://ja.wikipedia.org/wiki/Memories_Off%E3%82%B7%E3%83%AA%E3%83%BC%E3%82%BA%E3%81%AE%E7%99%BB%E5%A0%B4%E4%BA%BA%E7%89%A9#.E3.83.A1.E3.83.A2.E3.83.AA.E3.83.BC.E3.82.BA.E3.82.AA.E3.83.95_.E3.82.86.E3.81.B3.E3.81.8D.E3.82.8A.E3.81.AE.E8.A8.98.E6.86.B6.E5.88.9D.E5.87.BA.