Koko Rinne is a seventeen-year-old member of the group Majo no Musume, also known as the witchlings, who uses her singing magic to defend Kamitsubaki City from the tesseractors that threaten it. She is the daughter of a witch and lives on Fourth Avenue, but due to circumstances that remain unexplained, she has lost all memories of her past. This amnesia defines much of her early arc, as she gradually pieces together her history while forming new bonds with the other witchlings.

In appearance, Koko is calm and gives off a mature, dignified impression, but her true nature is noticeably clumsy and absent-minded. She is straightforward with her emotions, often becoming moved to tears easily and, conversely, displaying high-tension outbursts of energy. Despite her sometimes airheaded tendencies, she carries a strong sense of responsibility, thinking of herself as the oldest among the group and feeling a deep need to protect the others. This protective instinct is one of her primary motivations, alongside a desire to safeguard the city and to understand the secrets that lie behind her lost memories.

Koko occupies an important role within the witchlings as both a frontline combatant and a strategic monitor. During major crises, she often stays at the command center to track Incomprehensible Regions and coordinate responses, as seen when she monitors for outbreaks while the others engage the enemy. Her singing ability is particularly potent: she possesses a powerful, soulful voice that can stabilize the city’s core during extreme emergencies, and her songs are a key weapon against tesseractors. She also forms a notable partnership with Sekai Yorukawa, their voices complementing each other to produce a harmonized “Ensemble” effect.

Her key relationships include a mentoring dynamic with Kafu Morisaki, whose development she watches over and guides. She shares a close bond with the other witchlings—Rime Tanioki, Haru Asanushi, and Sekai—and often acts as an elder-sister figure. A significant connection exists with Kugel, a figure from her past whom she befriended after waking up with amnesia; his later betrayal weighs heavily on her. As the story progresses, Koko must reconcile the hidden knowledge she possesses about the city’s dark origins with her desire to protect her friends. Her personal journey involves moving from a mysterious, somewhat reserved presence to someone who openly embraces her emotions and her place among the witchlings, culminating in a major revelation or sacrifice that shifts the balance of power within Kamitsubaki City.