The daughter of Ai Hoshino and twin sister of Aquamarine Hoshino, Ruby is actually the reincarnation of Sarina Tendouji, a young terminal patient who was a big fan of Ai and developed a close friendship with her doctor Goro Amamiya, the previous life identity of Aqua. Initially bright and optimistic, Ruby's personality darkened considerably on her path for revenge, leading her to become two-faced and manipulative. After making peace with her past, she matures into an inspirational idol.