Kouhei

Kouhei's real name is Shintarou, but after he transforms into a child, due to some magical incident that he never finds the reason for, he takes on the name Kouhei. His younger sisters have always hated him, but the fall for him when they think he's somebody else, and he starts having sex with all three of them, Ayaka, Fuyuno, and Chiaki, on a regular basis, even after he transforms back.