Hayasaka Umi

Umi is the real sister of Akihito who died being hit by a car years ago, just a few days before the official adoption of Umi.

Her death shocked Akihito so much that his subconscious replaced the sister who had passed away by his adoptive sister. At the start of the game, he still thinking that Umi (羽未) is his real blood-related sister.