Imaizumi is Senri's "best friend" from primary school. He was a small and timid child who was bullied by other students. During this time he meets the class hero Senri who always helps him. So Imaizumi asks Senri to be his friend and thus they became friends.
However, during his childhood, Senri knew nothing but acting and because of this he thought they were just acting as friends. Imaizumi discovers this when Senri asks why Imaizumi always plays the bullied and never the hero.