Ikuto Tsukiyomi is a central figure in the story, a tall teenager with dark navy hair and indigo eyes who is often seen wearing dark, casual clothing, frequently including a choker with a silver cross. He is a skilled violinist, a talent inherited from his father, and his life is largely defined by the burdens placed upon him by the Easter Corporation.

His personality is enigmatic, aloof, and often described as cat-like—mysterious, a little naughty, and prone to teasing, especially towards the protagonist Amu Hinamori and his rival Tadase Hotori. Behind this mischievous exterior, however, lies a deeply selfless and protective individual. Ikuto is willing to distance himself from those he cares about, or even deliberately hurt their feelings, to prevent them from becoming entangled in his dangerous troubles.

Ikuto’s circumstances are complicated. He is the older brother of Utau Hoshina and is bound to work for the Easter Corporation, which he deeply despises, after his mother remarried the company's president. Easter has contracted him to find the legendary Embryo, a powerful artifact that can grant any wish. While he performs this task, he secretly hopes to use the Embryo to free himself and his sister from the company's control. A key part of this struggle involves his possession of the Dumpty Key, which is meant to pair with the Humpty Lock owned by Amu. His personal motivation is further driven by the disappearance of his father, Aruto, whom Ikuto refuses to believe is dead and has resolved to find.

Throughout the story, Ikuto’s role shifts from that of an antagonist to a crucial ally. Despite working for Easter, he often helps Amu and her friends, and his feelings for her deepen significantly over time. He confesses his love for her, though she initially believes it is just another one of his teasings. Ikuto's relationships are marked by sacrifice. His bond with Tadase is strained because Ikuto deliberately pushed him away years ago to keep him safe. Similarly, he maintains distance from his sister Utau to protect her from Easter's influence, even as she struggles with her own complex feelings for him.

Ikuto undergoes significant development, from a resigned pawn of a corporation to an individual actively fighting for his own freedom. In a moment of despair, he even surrenders himself to Easter to keep others from getting hurt. His internal darkness and trauma are physically manifested when he is manipulated into transforming into Death Rebel, a dark and powerful form he cannot control. However, Amu’s unwavering acceptance and feelings for him help save him from this darkness, leading to a pure, joint transformation called Seven Seas Treasure, which symbolizes his ultimate freedom. At the series' end, he leaves to search for his father but promises to return for Amu when she is older, solidifying his journey from a trapped loner to a man determined to build his own future.

Ikuto possesses several notable abilities. His Guardian Character is Yoru, a mischievous cat who embodies his desire for freedom. A standard Character Transformation with Yoru creates Black Lynx, which grants him feline agility, enhanced senses, and the attack Slash Claw. Under Easter’s control, he becomes Death Rebel, a transformation that allows him to use his violin to create devastating attacks, including a weaponized scythe. His ultimate form, Seven Seas Treasure, is a pirate-themed transformation achieved alongside Amu that wields the attack Emerald Line. Even outside of these forms, his very soul can manifest cat ears and a tail, and he can project energy claws known as Phantom Claw.