Umi no Yami, Tsuki no Kage

海の闇、月の影

Lumi and Luka Kobayakawa are identical twins who both are members of the track team and have a crush on a handsome upperclassman Katsuyuki Tohma, but he chooses the latter. While Luka doesn't want to hurt Lumi's feelings, Lumi feels betrayed by her own sister. On their way to the party celebrating the graduation of eldest members of the team, they take shelter in a cave, which turns out to be a mass grave, and all members were killed by the strange smell from the cave, with Lumi and Luka only survivors. After the tragedy, Lumi has become a completely different person, and one day she literally emerges from the floor of Luka's room... (Source: Anime News Network)