Mitsuki Watanuki serves as the youngest sibling among the three Watanuki sisters, distinguished by her violet hair and golden eyes. After losing both parents—her mother in early childhood and her father roughly one year before the main events—she resides with her sisters in their inherited family home located in Tokyo's Koto City, specifically within the Monzen-Nakacho district.

Her personality merges logical thinking with social difficulties. She perceives herself as cool and collected, routinely approaching scenarios through rationality. This logical focus occasionally disrupts her conversational flow when overanalysis takes hold, yet she maintains a steadfast sense of familial responsibility beneath her detached self-image.

Professionally, Mitsuki studies science and engineering at the collegiate level, recognized as a programming prodigy. She actively learns programming languages, including Chinese, to achieve her goal of founding an IT company. Her personal interests encompass YouTube and virtual YouTubers.

Emotionally, her mother's early death left enduring marks. Though seldom voiced, she carries a profound yearning for maternal affection and guidance, creating a vulnerability that contrasts with her otherwise rational exterior.