Nanaka 6/17

Kirisato Nanaka is a 17-year-old girl, and she is a 2nd year student in a high school. She has a childhood friend, Nagihara Nenji, who goes to the same school as her. However, because of an accident at one day, Nanaka lost her memory from when she was 6-year-old and onward. In other words, she mentally turns into her 6-year-old self, which has to be kept secret from all her classmates, of course. To make things more complicated, an honour student Amemiya Yuriko, Nagihara Nenji and his two rivals, Arashiyama Jinpachi and Arashiyama Satsuki, somewhow get tangled into Nanaka's school life. So what will happen to Nanaka?