Chika Yamada, the 12-year-old younger sister of the main protagonist, navigates adolescence with a maturity and social awareness that starkly contrasts her sibling’s chaotic, hypersexualized pursuits. A middle school student, she leverages inherited charm and calculated innocence to manipulate boys into securing gifts or favors, cementing her popularity and inadvertently overshadowing her sister’s efforts. Her calm, pragmatic approach to relationships—dating older boys without theatrics—amplifies her sibling’s insecurities, as Chika’s strategic "Lolita charm" highlights her effortless command of social dynamics. Yet she remains detached from overtly sexual interests, often responding to her sister’s antics with bemusement or quiet disapproval.

Though perceived as romantically experienced, Chika admits to lacking firsthand knowledge when asked for advice on intimate matters, revealing a divide between her social manipulation and genuine intimacy. Her intuitive grasp of emotional undercurrents allows her to dissect her sister’s romantic struggles with observational precision, though she intervenes only when prompted. This duality underscores her role as an inadvertent guide, balancing youthful image with shrewd insight.

Visually, Chika’s black pigtails, cyan eyes, and petite 1.50-meter frame craft an image of deliberate innocence, aligning with her use of youthful charm as a social tool. Her presence accentuates contrasts between naivety and sophistication, dissecting adolescent hierarchies through subtle influence rather than direct action. While secondary in narrative focus, her effortless success repeatedly destabilizes her sister’s self-perception, reinforcing thematic tensions around rivalry, perception, and the quiet power of calculated charm.