Ikuto Tsukiyomi is a central figure known for his complex and often antagonistic role. He is a teenage boy, typically presented as being between fifteen and seventeen years old, with a tall stature, blue hair, and blue eyes. His appearance is frequently marked by dark clothing and a distinct choker bearing a silver cross, often accompanied by a signature smirk that conveys both amusement and mystery.

His background is dominated by a troubled family history tied to the Easter corporation. Ikuto is the son of a renowned violinist named Aruto, from whom he inherited both his musical talent and the Dumpty Key. After his father disappeared and was presumed dead, his mother remarried into the leadership of Easter, which effectively bound Ikuto and his younger sister, Utau Hoshina, into servitude for the company. This forced arrangement requires him to hunt for the Embryo, a legendary Guardian Egg capable of granting any wish, a goal he despises but pursues in the hope of using the wish to free himself and his family.

Personality-wise, Ikuto is consistently portrayed as aloof, enigmatic, and independent, carrying himself with the untamed air of a stray cat. He is known for a mischievous and sometimes naughty streak, taking great pleasure in teasing other characters, particularly the protagonist Amu Hinamori and his rival Tadase Hotori. Despite this playful exterior, he is deeply selfless and protective, willing to alienate himself and bear the hatred of others to keep them from being entangled in his dangerous affairs with Easter. His childhood personality was quiet and shy, and a more sincere, vulnerable side emerges primarily in his interactions with Amu.

Ikuto's role in the story is that of an anti-hero. While ostensibly working for the antagonistic Easter corporation, he frequently acts against their interests by helping Amu and the Guardians. His central motivation is the desire for freedom from Easter and the hope of finding his missing father, whom he refuses to believe is dead. His character development is heavily tied to his relationships, especially his growing romantic feelings for Amu Hinamori. He eventually confesses his love for her, though his teasing nature often makes his true feelings ambiguous to her. In a pivotal act of sacrifice, Ikuto surrenders himself to Easter to prevent others from being harmed, and later leaves to search for his father, vowing to one day return for Amu.

His key relationships involve his sister Utau, whom he keeps a distance from to protect her from Easter; Tadase, with whom he shares a deeply strained rivalry rooted in childhood misunderstandings and competing for Amu's affection; and Amu herself. His bond with Amu is symbolized by their paired items: his Dumpty Key and her Humpty Lock, which metaphorically unlock each other's hearts.

Ikuto possesses several notable abilities. He is a highly skilled violinist, a talent inherited from his father that serves as an emotional outlet and, at times, a weapon. His primary power comes from his Guardian Character, Yoru, a cat-like being. With Yoru, he can Character Change to gain cat-like traits such as ears and a tail, and he can perform a Character Transformation into Black Lynx, which grants him enhanced agility and the ability to use attacks like Slash Claw or Phantom Claw. Under Easter's manipulation, he is forced into a corrupted transformation called Death Rebel, where he uses his violin to control Heart's Eggs. Later, through his deepening connection with Amu, he achieves a more powerful and pure transformation known as Seven Seas Treasure, representing his ultimate freedom.