Azuki-chan

Azusa Noyama is a 12-year-old girl student, taking 5th grade, but everyone knows her as Azuki-Chan, nickname that she dislikes. Yuunosuke Ogasawara, a new student, asks her about her nickname, saying that he likes it. Azuki falls in love with him, and they start going out together. They share a diary, and become boyfriend and girlfriend.

(Source: Anime News Network)