Leiji seems to have some problems with Kanade, saying that he will never accept her existence and everything about her. He appears to know Kanade from before, but she doesn't seem to remember him. He tells her that she humiliated him in the past.
He holds his violin very dear, and even punched Kyoya when he was about to touch his violin.
He knew Kanade 7 years prior to the start of the original series, as they were rivals competing in the individual concours as children.As seen in the flashbacks, young Leiji's first encounter with Kanade was when he saw her crying and upset over the fact that one of her violin strings had broken, which meant that she couldn't participate in the concours as she did not even have a spare string with her. He gave her a golden E string in addition to helping her tune her violin. She thanked him for his kind gesture, to which he felt slightly embarassed.
For the concours, both Leiji and Kanade happened to play the same piece, however her genuine playing skills and talents were much more apparent to the point where even Alexei had acknowledged her abilities and foreshadowed her winning the competitions. He made a promise with young Leiji before the concours that if he managed to win, he would get any of his wishes fulfilled, including the one involving his younger sister, Shiori, but after hearing young Kanade skillfully play her violin, he concludes that there isn't any chance for Leiji to win the competitions as a winner has already been decided (Kanade). Leiji requested Alexei to stay back for the results as he believed he could still make it, however, Alexei presents him with a harsh truth which Kanade happens to eavesdrop on. She was terrified and understood Leiji's predicatement which led her to forfeit the competition for his sake.He gets so upset with her that he taunts her by saying that he would hate her for the rest of his life for insulting his music and his existence, which he quotes as saying "you gave me the victory out of charity, which is like throwing a bone to a stray dog". He then tells her that he won't forgive her, and that he will drag her down to hell. Hearing him speak such harsh words came as a shock to her, which frightened her to the point where she collapses later and subconsciously erases the very last traces of memories she had with him in their childhood.
Later on he and Kanade got kidnaped by Takafumi. He kidnaped them because he wanted the attention of Leiji and wanted to play in the concours with him. But it didn't turned out the way he wanted. Leiji insulted him and Takafumi got mad, accidantly hitting Leiji and afterwards he ran away closing the door behind him and leaving Kanade and Leiji locked up in the room. But Sei manages to find out where they are and told everybody about it. Afterwards, Leiji and Kanade managed to get out and the both of them made it to the concours in time.
At the end of the concours, Seiso ends up winning and Leiji was 'pulled out of the darkness' by Kanade and he called her his 'femme fatale'.