Mutsuki Nakano is the oldest sibling in the Nakano family, the central character from the comedy slice-of-life series that follows the daily lives of five siblings living together in an apartment complex. She is a high school student who excels in multiple areas, including cooking, academics, and sports, and is regarded as the perfect older sister by her younger siblings and peers at school. However, this composed and responsible public image is a carefully maintained facade. Mutsuki possesses a contrasting private side that she reveals only to her younger brother, Haruki: she is noticeably more relaxed, nonchalant, and willing to drop her formal demeanor around him, often slipping into his room to rest or act more casually. This duality is a defining trait of her personality and a source of comic and character-driven moments in the story.

Mutsuki's primary motivation is to maintain harmony and order in the household. She serves as a stabilizing force among the siblings, often stepping in to mediate minor conflicts, enforce discipline such as threatening to cut allowances, and manage household tasks like grocery shopping. Her protective and authoritative role is tempered by a genuine warmth and care for her siblings, though she tends to express it through actions rather than overt affection. Her character is largely static rather than undergoing dramatic change; her development centers on the gradual revelation of her private self to the audience and her brother, highlighting the gap between her public perfection and her more human, laid-back tendencies.

Key relationships define her role in the story. With Haruki, she shares the unique bond of being the only person who sees her true self, which creates a dynamic of playful secrecy and mutual understanding. With her younger sisters Yayoi, Uzuki, and Satsuki, she maintains a more formal, responsible older-sister persona, using her authority to guide and sometimes scold them. The twins, Uzuki and Satsuki, perceive her as scary but also acknowledge her kindness. Mutsuki's abilities include high-level proficiency in cooking, which she uses to prepare meals for the family, as seen when she secretly swapped a spicy curry mix for a mild one to avoid disappointment. She is also academically capable but a poor teacher, a trait that Haruki notes when she offers to help with homework. Her athletic skill is mentioned as another forte. These abilities, combined with her dual-layered personality, make her a central figure in the household's comedic and heartwarming interactions.