Hiiragi Keiichi is a prominent human character who begins as an antagonist before shifting to a supporting role. He is a tall senior high school student with bluish-gray hair and gray eyes, traits not inherited from his mother but presumably from his father. As a famous violinist and teen heartthrob, he lives in a large mansion managed by his butler, Sebastian, and is the grandson of the Hiiragi Corporation. While he possesses good grades, athletic skill, and proficiency in aikido, he is often indifferent to his popularity among female students, finding the attention tiresome.

Keiichi’s personality is defined by a cool, aloof, and freedom-loving nature, coupled with a deep sense of boredom and emptiness. Having been blessed with immense talent and wealth, he lost his interpersonal connections and his passion for music, leading to a state where he had given up on his dreams entirely. He feels trapped by his own abilities and the expectations placed upon him. Despite this cold exterior, he possesses a highly sensitive disposition, as demonstrated when he fainted after hearing a classmate's surrealistic poetry. He is initially willing to use subterfuge to achieve his goals, including feigning a romantic interest in Uta Yumeno to interfere with My Melody, though these actions later become complicated by genuine feelings.

Motivated initially by sheer boredom and a desire to find a hidden, unheard melody, Keiichi agrees to help Kuromi. By playing his violin to supply Dark Power to Kuromi's Melody Key, he seeks to fill the void left by his lost aspirations. As the designated wielder of the Melody Bow, his music is a primary source of dark energy in the first season. Throughout the story, Keiichi struggles with his feelings for Uta Yumeno. For a long time, he misinterprets his romantic love for her as the affection one would have for a younger sister, a realization that only fully dawns on him later. He also grapples with the pressure of his musical career, attempting to quit or destroy his violin twice and succeeding in breaking it once.

As a consequence of his actions in the first season, Keiichi is punished by being forced to transform into the Bunny Eared Mask (Usamimi Kamen) whenever My Melody summons him for help. He deeply hates this role, often appearing lethargic and irritated, and dislikes My Melody for forcing him into the humiliating rabbit costume. Despite his resentment, he fulfills the role throughout subsequent seasons, with his identity eventually being revealed to the main characters. He shares a complex relationship with his younger brother, Jun Hiiragi. Initially cold and dismissive of Jun's talents, Keiichi’s attitude causes a rift that allows Jun to be controlled by Dark Power. Keiichi eventually reconciles with his brother by sincerely apologizing and playing a duet with him.

Keiichi's development is marked by his gradual emotional awakening. Initially numb and dreamless, his interactions with Uta and even his forced heroics as the Bunny Eared Mask force him to confront his feelings. He eventually realizes that his true dream involves both his violin and Uta. While he finally acknowledges his love for Uta, her affections have shifted to another, and she ultimately sees him as a brother figure, leading to a bittersweet resolution. By the end of the series, he resumes his music career with renewed purpose and goes abroad to study with his brother. As a testament to his abilities, he is highly skilled in playing pieces like "Zigeunerweisen," possesses absolute pitch, and can manipulate dark power through the strength of his emotions and mental state. His connection to Kuromi is also notable; while she harbors a strong crush on him, he shows occasional affection for her human form, Kurumi Nui, creating a complicated bond between them.