Mutsuki Kururugi is a 16-year-old male student at Étoile Vio School who forms the idol unit Twinkle Bell with his fraternal twin brother, Satsuki. Standing 172 cm tall and weighing 53 kg, he has wavy pale beige hair accented by a sky-blue highlight, pale blue eyes, and pink-rimmed glasses. His typical attire includes fingerless gloves, a jacket, and a belt.

Mutsuki exhibits a relaxed, lethargic personality that starkly contrasts Satsuki’s energy. He frequently drifts asleep mid-conversation or activity, moving through life at his own unhurried pace. Despite this airheaded demeanor, he assumes a protective role as the older twin, tolerating Satsuki’s pranks but stepping in when they go too far. This dynamic originated in childhood, when a weak constitution confined his social world largely to Satsuki, creating initial emotional distance from others outside their bond. He later develops fondness for a fellow student, Futami.

His hobbies include cooking and napping. He excels at preparing fried eggs and sweets, skills honed from regularly cooking for Satsuki. Cereal ranks as his favorite food, while he avoids chicken skin. Were he not an idol, he would pursue becoming a pastry chef. Satsuki curates his wardrobe, and an amusement park they visited together holds deep childhood significance.

Mutsuki joined idol work solely at Satsuki’s request, feeling obligated to support his younger brother. In Twinkle Bell performances, he emphasizes their tight synchronization. Personally, he harbors simple aspirations, often wishing to nap endlessly in the "dream world."