Koruru is a young mamodo, a childlike being from another world who is sent to Earth to take part in a tournament to determine the next ruler of her kind. Unlike most mamodo, she has no desire to fight. She is found alone and crying in the streets of Mochinoki by a teenage girl named Lori, who takes her in and treats her like a younger sister. Lori makes a special doll for her called Tina, and the two develop a deep bond. Koruru is gentle, kind, and innocent by nature, but she carries a tragic duality: when Lori reads a spell from her pink spellbook, Koruru transforms into a violent, battle-hungry alternate personality. This second self has no memory of its actions and is fiercely aggressive, using powerful claw-based attacks. Koruru herself is unaware of these transformations and is horrified when she sees the destruction she has caused.

Her meeting with Zatch Bell and his partner Kiyo Takamine is a turning point. Even in her confusion and fear, Koruru expresses a sincere wish for a kind king who will end the fighting so that mamodo no longer have to hurt one another. This simple plea becomes a core motivation for Zatch, shaping his entire goal throughout the tournament. Koruru ultimately chooses to have her own book burned by Zatch, voluntarily returning to the Mamodo World to prevent herself from being forced to fight again. This act of sacrifice is selfless and matures her character from a frightened child into someone who takes control of her fate. Later, at a critical moment in the final battle, Koruru returns as a spirit to aid Zatch and Kiyo, granting them the protective spell Shin Raifojio, which shields them from harm and allows them to survive in a hostile environment.

Koruru's abilities center on body enhancement and transformation. Her known spells include Zerusen, which extends her arms to strike; Gigano Zerudo, which causes claws to erupt from the ground; Raja Zerusen, a more powerful version of Zerusen; Zerarusen, a claw-based attack; Zerudo, a defensive aura; and Zeruk, which augments her already dangerous battle form. Shin Raifojio is a rare healing and protection spell she uses in her spiritual return. Despite her short time in the story, Koruru's impact is lasting. Her relationship with Lori remains the emotional anchor of her character, and her influence on Zatch's resolve to become a benevolent king reverberates throughout the entire series. Her development is marked by her transition from a reluctant participant to a willing sacrifice, and finally to a guiding presence in the climactic struggle.