Len Tsukimori is a second-year violin student in the Music Department at Seiso Academy. Hailing from a family of renowned musicians—his father a prominent violinist and his mother, Misa Hamai, a world-famous pianist—intense pressure shaped his youth. This drove him to practice at least eight hours daily, fostering a perfectionist mindset where music meant only competition and technical mastery. He found no inherent joy in performance, maintained a cold, distant demeanor, and consequently had few friends.

His worldview shifts upon encountering Kahoko Hino, a General Education student who receives a magical violin and enters the school's music concours. Initially dismissive of her abilities and skeptical of her sudden prowess, Len is unconsciously stirred by her passionate, heartfelt approach to music. Hearing her play reveals the emotional depth missing from his own performances. He gradually warms to Kahoko, offering unsolicited technical advice and eventually agreeing to tutor her formally. His mother observes positive changes in both his playing style and personality, attributing this growth to Kahoko's influence.

As their relationship deepens, Len struggles to acknowledge his feelings for her. He grows envious of her interactions with other competitors and becomes visibly flustered in her presence, though he initially dismisses these emotions. His internal conflict peaks when he decides to study abroad in Europe but delays informing Kahoko until the eve of her performance. After encouragement from Ryotaro Tsuchiura—who confesses his own feelings for Kahoko—Len admits his love to her. In the manga, he explicitly states, "My feelings for you... if expressed in words, then it is Love," leading to a mutual confession and embrace. Their relationship endures in later media, with Len referencing Kahoko's insights when interacting with newer characters.

In later installments like *La Corda d'Oro: Blue Sky*, Len appears as a cameo, now a professional violinist. His appearance matures, featuring darker blue hair and a more open demeanor. He plays "Ave Maria"—a piece significant to his bond with Kahoko—in locations meaningful to their past, such as her former practice spot. In one episode, he advises a younger musician, Kanade Kohinata, on the importance of emotional unity in ensembles, quietly attributing this perspective to Kahoko's influence years earlier. This illustrates his enduring evolution from a technically driven performer to a musician valuing emotional connection and mentorship.

Beyond violin, Len possesses piano skills, likely from his mother's influence, and proficiency in English and German. He struggles with non-musical activities like cooking and has limited experience with everyday Japanese culture, having never tried common street foods. His personal evolution spans the series, culminating in a mature artist balancing technical excellence with emotional sincerity.