Bokura ga ita (2006)

Nanami Takahashi is a high school student entering her first year. Immediately on her first day however, she becomes the victim of a practical joke by the school's most popular guy, Motoharu Yano, who coincidentally also ends up in her class. While the two get off to a rocky start, they soon find themselves falling in love with one another and begin dating. But their relationship is marred by Yano's past which includes the death of Yano's previous girlfriend, Nana Yamamoto, and the betrayal surrounding her death. To make matters more complicated her younger sister, Yuri, is in the same class as Yano and Nanami. As Yano struggles to come to grips with Nana's death and his unresolved feelings for her, so too must Nanami learn to understand Yano if the two hope to continue their relationship with one another.