Sui is Kei's younger brother and resembles a younger version of Kei. Initially, he dislikes Kei because he finds living with him disgusting and thinks of him as a robot wearing human skin. Because Sui feels very inferior to his brother, who is good at everything, he believes that he cannot reach Kei's level and he refuses 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 Hakusenkan's elementary school division.

Source: Wikipedia