That Summer is Saturated.

One summer day, a junior high girl named Ryuka accidentally kills the girl who was bullying her. When Ryuka said she is going to go somewhere far away to die, Chihiro told her, "Take me with you," and the two set off on their escape. However, when the police caught up with them, Ryuka left Chihiro behind and took her own life. Thirteen years later, Chihiro is an adult, but he is still unable to forget Ryuka, and he lives his life in apathy. Until Ruka, a high school girl who looks very much like Ryuka, comes into his life.

(Source: Kadokawa, translated)