Chii-chan wa Yukyu no Muko(2008)

Chigusa and Yuuto always spent time together as children. They enter high school at the same time and happen to be put into the same class. Yuuto's mother ran off and his father is an alcoholic, so it seems as though the only time he can feel any relief from the pain of his life is when he spends time with Chii-chan and can be reminded of the happy little moments from his past.