Len's relationship with Kahoko Hino started out rocky. He behaved harshly towards her and on many occasions, was suspicious of her violin skills after she had claimed she was a beginner. Len often made her uncomfortable and wondered whether she was "the real deal". However as the series progresses, Len starts to warm up towards Kahoko. Deep down in his heart Len is concerned about her although he does not show it. 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. Kahoko showed Len the true meaning of music and this is what drew him to her.
Len usually did things for Kahoko that he wouldn't normally do for others. Like in Chapter 35 in the manga, he agrees to accompany her to the amusement park along with Ryotaro Tsuchiura and his ex-girlfriend. When Kahoko gets too scared in the haunted house, Len offers to hold her hand throughout the way. Another instance is when he only invites her to especially watch him perform at a concert with his mother. He gets annoyed on finding out that his mother had invited the other participants of the concours as well.
Len even begins tutoring Kahoko seriously on how to play the violin well and goes on to drop her home after their sessions.
In chapter 63, Len finally realizes his true feelings for Kahoko that seems to be confusing him for a long time. He realizes that his feelings for her were "love". In chapter 65, Len decided to go along with the group to watch Ryoutaro's concert. While all of the people are watching Ryoutaro, Len only stares at Kahoko. This is noticed by Azuma Yunoki who tells Len that he's being obvious.
In the end he confesses that only Kahoko can make him feel the way he does and says only she can "perturb his heart" like that and they embrace. This is the manga finale making him in one way the official love of Kahoko. Both he and Kahoko are considered the official pairing in the manga and by many fans. This was further made canon by the Special Edition CD where, after you get all character ends (as in play all character routes), you get a special end which is an anime section that follows the style of the anime series and can be considered a part of it as well. There Kahoko is outside the music room and sitting on the grass listening to Ave Maria. As Len finishes playing he actually is very happy to see her (a great change compared to how he treated her the first time they met). Kahoko sits in on the window on an angle still outside and Len also sits but facing in. He reminisces and an inner monologue begins about how Kahoko is so precious to him and what she has taught him, and how much he truly loves her (while important images are flashbacks from the anime). He touches Kahoko's hand and Kahoko does not retreat but blushes and smiles making him smile too (as if he was hoping for a positive and accepting response from her). Soon he softly edges forward, Kahoko looks surprised and shy, but closes her eyes allowing him to advance further. Here Lili comes out from behind Len's golden violin and decides to put magic dust on the moment to freeze it. Len has all this time still been talking inside exploring the depth of his love and feelings for Kahoko. The end of the entire series finishes with him and Kahoko edging closer to consummate their love with a kiss.
Len and Ryotarou never did like each other from the start of the series. They argue about everything; from a lost notebook to their performance styles. Len consistently looks down on Ryotarou for being a General Ed student and this is usually the cause of many of their arguments.
When the six participants go to training camp, Len and Ryotarou are assigned a room together, which only spells disaster and both roughly approach Mr Kanazawa to change their rooms the next morning.
Ryotarou seemed to sense Len's growing feelings for Kahoko. He made Len aware of these feelings on numerous occasions: The first being when he asked Len why he'd told everyone else about his overseas plans but not Kahoko. The second was probably the most daring: Ryotarou practically confessed his own feelings for Kahoko to Len. Doing so sparked a lot of thought for Len afterwards.
When Kahoko is set to perform in chapter 69 of the manga, Ryotarou notices Len's absence. Knowing that it would upset Kahoko, he leaves to get Len to attend and meets him in a flower shop. Len is persuaded by him and they both reach to watch the end of her performance.
Ryotarou usually appeared to be the voice of reason for Len, who, in turn would somewhat listen to him as well.
Misa Hamai Edit
Len and his mother, Misa Hamai had a rather formal relationship. Misa is definitely aware of Len's attitude to music and comments on how he takes everything too seriously. She worries about her son's inability to feel for anything. Misa soon notices Kahoko bringing a change in him and she happily acknowledges it.
Being the introvert that he is, Len doesn't have much of a relationship with anyone else other than the participants of the concours namely Azuma Yunoki, Kazuki Hihara, Keiichi Shimizu and Shoko Fiyuumi. He is usually seen hanging out with them(mostly against his will) numerous times.