Mitsuki Watanuki, aged 20, is the youngest of the three Watanuki sisters. Having lost her mother in early childhood and her father just one year before the main events, she and her sisters inherited the family home in Tokyo's Monzen-Nakacho district. As the youngest when their mother died, Mitsuki harbors a significant longing for maternal affection, though this need rarely surfaces explicitly in her daily behavior.

She exhibits a cool, logical demeanor and considers herself the most level-headed sister. A prodigy in programming, she attends college for science and engineering while actively studying Chinese alongside coding. Her career goal involves establishing her own IT-related company. Despite her self-assured attitude, her tendency toward overthinking frequently disrupts conversations, causing her to lose track mid-way. This analytical mindset makes social interactions challenging.

Her interests include watching both YouTubers and VTubers, aligning with her tech-oriented pursuits. Within the family dynamic, she demonstrates the strongest sense of responsibility among the sisters, often contrasting with their differing personalities. While she presents herself as composed, her underlying vulnerability surfaces through her unfulfilled need for motherly care, a consequence of her early loss.