Leiji seems to have an unsettling hatred for her, due to an incident that happened between them in the past. His only desire now is to crush Kanade in the concours, so as to get avenge himself for what she did to him in the past.
It is later revealed in Episode 11 that the main reason for him wanting to gain his revenge on her was because she intentionally allowed him to win when they were competing against each other as children, though she had the upper hand in terms of skills and talent, thus giving him an easy win. Her act of consideration was viewed by him as an act of hostility as he wasn't very satisfied with the win.
When they were trapped in a basement in Episode 11, Kanade remembers the face of the boy that had given her the golden string and had helped her tune her violin when they were young, which turned out to be Leiji. She also discovers that he was the one who wrote her the letter which read - お前はここで終わるのか "Are you done for, here?" (or "Have you reached your limit?")
Inspite of making his hatred apparent to her in terms of words, he doesn't truly hate her, as seen when he protects Kanade from an unstable Hido, when he (Hido) was about to hit her with a steel pipe.
He is Leiji's adoptive father. He takes him in when Leiji's parents died in a car accident several years before.
She is Leiji's younger sister. She constantly worries about her elder brother and considers Kanade to be the only person who can save him from the darkness which he has engulfed himself in.