Yomogi Tanaka is the younger twin sister of Mizuna Tanaka and the youngest sibling in the Tanaka family, one year junior to her older brother. She attends the same junior high school as a first-year student. Physically, Yomogi possesses long black hair with the distinctive Tanaka family hair tuft, thick enough to hold tennis balls. Her black eyes and notable height surpass both her twin sister and other girls at school. She wears the standard blue first-year uniform with black tights and changes into the school gym outfit for tennis activities.

Emotionally sensitive and more childish than her twin, Yomogi cries easily and frequently seeks physical reassurance from her brother, Tanaka, during distress. Her recent growth spurt complicates this comfort ritual. She exhibits affectionate curiosity, such as approaching Yamashita of the Science Club, and favors mint-chocolate flavor.

Yomogi participates in the school's Tennis Club under President Minamine. Her interactions with Ueno occur through club incidents or Science Club visits with Mizuna, remaining unaware of Ueno's romantic feelings toward her brother.

Her documented narrative appearances begin in Chapter 30, tasting Ueno's "Peroririon" invention. In Chapter 35, she voices childhood worries about leg hair. Chapter 37 features her explaining Minamine's dislike of boys. During a Science Club investigation with Mizuna in Chapter 42, her twin shields her from the clubroom's interior. Chapter 53 highlights her identifying the scent of Ueno's "Reflecushion" as matching Mizuna's home cushion. Minamine's yelling reduces her to tears in Chapter 66, prompting Tanaka to attempt holding her—a moment witnessed by a returning Minamine. Chapter 74 depicts Yomogi collaborating with Tanaka to confront Mizuna about not wearing panties, leading to a familial "trial."