He is a violin prodigy. Born in a family of musicians, Len feels overshadowed by his parents and strives for perfection in his music. He holds a high standard and demands the best from himself and from others.
Others may think that he is acting harshly towards Kahoko, but deep down in his heart he is concerned about her although he does not show it. As the series progresses he realizes his feelings of love for Kahoko. It becomes more obvious in the more recent chapters of the manga. For instance, while everyone else knows that he is leaving to study in Europe, he finds it the hardest to tell this to Kahoko and he doesn't really understand why, instead claiming the fact that his leaving has nothing to do with her.
It takes Ryoutaro, who confesses to him that he (Ryoutaro) likes Kahoko, to make him open his heart and acknowledge certain things regarding her. It's also seen that it is only around her that he blushes furiously later on in the series - for instance, when she held his hands, and when he walked her home. In chapter 63 of the manga, he becomes fully aware of his feelings when he thinks "My feelings for you...if it is expressed in words, then it is Love." Len's father is a well-known violinist and his mother is a famous pianist called Misa Hamai. He is fluent in both English and German.
Yunoki finds out that Tsukimori has feelings for Hino. When Tsukimori didn't want to tell Hino that he was going to leave in a week, Tsuchiura was so angry because Len was being cruel to Hino. Len told him that even though he confessed to her, it would not change her performance during the contest. On the day of Hino's contest, he didn't managed to watch her first performance, making her nervous to perform without seeing him at the audience's seats. So Tsuchiura went out to find Tsukimori, to get him to attend Hino's performance. At last they both managed to hear her perform her last piece.
Though the anime somewhat reflects on it the ending of Len and Kahoka's relationship is seen in the manga and Special Edition CD which included an anime segment. In the manga Len thoroughly confesses that only Kahoko can "perturb his heart" after Kahoko rushed to the roof in anticipation in seeing him so intensely that it confirms her own love for him. Len then smiles sweetly and embraces Kahoko as his final confession of love. In the anime segment Len is playing Ava Marie and Kahoko hears him. He is very happy to see her (showing from where their relationship started from him being distant and a bit moody to how it has become warm, respectable and filled with mutual love). Soon they on the window and Len goes through images in his head and a very heartfelt internal monologue on how much he loves Kahoko and how much she means to him. Soon he touches her hand and Kahoko reciprocates his feelings with a blushing smile. Len smiles too and edges nearer. Kahoko is a bit flustered but allows him to advance as they mutually get closer. Lili comes from behinf Len's golden violin and seemingly freezes the moment with magic dust. Len is still monologuing inward at his expanse of feelings for Kahoko. As they consummate their love with a kiss.
Len and Kahoko future and plot revolves around each other. This was made canon by all media. It seems they both grow as musicians and individuals because of the depth of love, respect and admiration they develop with each other as the story progressed. In Kiniro no Corda they are the official couple.