Sui is Kei's younger brother and resembles a younger version of Kei, which leads Hikari to call him "Little Takishima," when she first sees him. At first in Episode 4 Sui hated Kei but in the end they got along.
Initially, Sui disliked Kei because he found living with him disgusting and thought of him as a robot wearing human skin. Because Sui felt very inferior to his brother, who is good at everything, he believed that he could not reach Kei's level and he refused to take his studies seriously. However, when he sees that even his "perfect" brother can become desperate and caring, Sui develops a huge brother complex. Sui likes to call Hikari "Stupid Girl", but approaches her frequently when he thinks that Kei needs help. He is in grade 6, class B at Hakusen Academy elementary school division.