Mizuna Tanaka stands as the elder non-identical twin sister to Tanaka, though she is shorter than her sister Yomogi. She shares the family's distinctive genetic hair tuft and possesses black eyes. Mizuna wears the standard blue first-year middle school uniform with knee-high socks, often choosing shirts printed with characters or numbers like "3", "R", "1", and "P". Outside of school, she sometimes appears in her swimming club jacket and swimsuit.

Compared to Yomogi, Mizuna displays a more temperamental personality. She's quick to yell at Tanaka when he acts harshly or strangely. A recurring comedic trait involves her opening the science club door at awkward moments, accidentally catching Tanaka in compromising situations with Ueno, which spurs her to investigate the science club's activities. Her favorite food is wagashi filling.

Mizuna debuted in Chapter 30 sampling the "Peroririon" invention. Key appearances include: retrieving Tanaka's thermos during Ueno's Sqool Swimsuit incident (Chapter 33), discussing leg hair with Tanaka (Chapter 35), investigating the science club with Yomogi and witnessing Tanaka holding Ueno's Mochi Bra (Chapter 42), reacting to the unsettlingly familiar scent of the Reflecushion (Chapter 53), and facing a mock trial resulting in a 405-year prison sentence contingent on underwear compliance after being accused of not wearing any (Chapter 74).

Her relationships include being classmates with Yamashita. She experiences typical sibling dynamics with Tanaka, who disciplines both her and Yomogi for causing trouble but also shows care, such as comforting Yomogi when scolded. Mizuna remains unaware of Ueno's romantic feelings for Tanaka.