Mika Nogizaka, Haruka Nogizaka's junior high second-year sister, possesses a personality sharply contrasting her older sibling's gentle naivety. She exhibits a mischievous, assertive, and perceptive nature. Upon meeting Yuuto Ayase, she integrates him into the family, affectionately dubbing him "Onii-san" and frequently teasing both him and Haruka about their relationship. Mika actively spies on the pair, often with the maid Nanami, fulfilling her self-appointed role to observe and encourage their romantic progress.

Despite her youth, Mika displays greater knowledge of romantic and sexual topics than Haruka, though explicit material still shocks or stuns her. Energetic and playful, she enjoys flirting with Yuuto and can become clingy when seeking his attention. At her all-girls school, Mika holds significant social standing as an idol nicknamed "Moonlight Strawberry" and has run for student council president two consecutive years. Her hobbies include playing violin, playing squash, caring for wild pigs, and spying on her sister.

Her name combines the Japanese characters for "beautiful" (美) and "summer" (夏). Mika develops romantic interest in Yuuto following a date but ultimately steps aside, acknowledging his relationship with Haruka. Her efforts to push them together stem from genuine care for her sister's happiness. Mika harbors a comical yet intense fear of ghosts.