Manga Hakoniwa No Soleil

After visiting her deceased teacher's grave, a guy suddenly falling of the staircase where Asahi Sena ended up saving him. That guy, Itsuki Tobari turned out to be her deceased teacher's younger brother. They started to talk about things from the past, about the dead of Itsuki's sister and an 11-year-old boy who killed her, who was Sena's bestfriend. What happened to the boy who killed her now? Together, they start to find out what did actually happen during that tragic day.