He is first seen when Kanade, Haru and Kyoya go to Amane High School. He stops the fight between Sousuke and Takafumi. Later on he meets Kanade again at a park where he is playing on a little piano for some children. Afterwards they go to play a duet where both she tells him that the piece sounded wonderful, but despite this, he tells her that his music is still missing something (love) as he cannot understand what love is. He decides that if he ever fell in love with Kanade, he might know what it means to put one's love into doing something, for this reason, he wants to actually try falling in love with her.
Later on he also notices that Leiji really dislikes Kanade, but also sees that Leiji shows more emotions when he is with her and he wants to figure out what their relation is.
Later on Sei gets dropped out of the concours, because he would be leaving to study overseas. But Leiji won't allow him to leave.
In Episode 11, when he was about to leave to the airport, he changes his mind and drives straight to the Linden Dorm asking for Kanade. When he meets Wataru and Iori, he learns about Kanade being kidnapped and suspects Hido. On realizing that something terrible may have happened to her, he looses his composure for the first time and frantically rushes over to the place where he believes Kanade and Leiji were being held as hostages, all this while, questioning his behavior as to why he would get so worried & disturbed over a missing person. It is implied that at this point, he may have started to develop feelings for her.