Miharu Mikuni is a high school student, first introduced as a third-year middle school student who later becomes a classmate of Keita Suminoe. Their first significant interaction occurs when she returns his misplaced high school acceptance letter, which her dog, Kenta, had taken. Overwhelmed with relief, Keita hugs her spontaneously, an act that sparks her initial attraction to him.

She has short blue hair and blue eyes in the anime adaptations. Nearsighted, she consistently wears glasses. She is distinguished by her F-cup bust, which is larger than that of other female characters in the series.

Her personality is quiet, meek, easily intimidated, and prone to anxiety, leading her to avoid confrontations and maintain distance from Keita, largely due to his stepsisters' aggressive advances and her own inexperience with romance. She suffers from a weak bladder, resulting in several instances of public incontinence that cause her significant humiliation.

Despite her reserved nature, she develops complex feelings for Keita that evolve from platonic into a strong romantic and sexual attraction. She often fantasizes about non-consensual encounters with him, revealing a specific fetish. This internal conflict between her desires and her shy demeanor sometimes manifests as an alternate, more aggressive personality called "Lewd Mikuni," which emerges when her anxieties spiral out of control. Only Keita can calm this personality.

Her relationship with Keita remains mostly platonic for much of the series, though she frequently becomes accidentally involved in compromising situations with him, often as an unintended casualty of his stepsisters' schemes. After Keita's relationship with Yūzuki Kiryū ends, Miharu becomes more proactive in pursuing him, including kissing him in front of their class after he mistakenly reveals his single status.

Her family includes a mother, a younger brother named Kouki, and her occasionally disobedient pet dog, Kenta. Though her familial relationships are not deeply explored, she comments that her personality is more like her father's than her mother's or brother's.

Her role varies across media. In the manga, she is introduced later, works as a part-time shrine maiden, and is a member of the school's volleyball team. In the anime adaptations, she is introduced earlier, her role is expanded to include more frequent accidental erotic encounters with Keita, and she serves as a school librarian.

Her character development shows a gradual shift from a passive, anxious individual to someone more confident in acknowledging and acting on her desires for Keita, even when facing competition from his stepsisters.