Tadakuni's younger sister—occasionally named Mei—attends Central High as a first-year while her brother studies a year ahead at Sanada North High. Their strained relationship involves minimal communication and frequent physical clashes, typically initiated by her. Physically petite with light brown hair in twin tails secured by pink bands, she often wears her school uniform layered with a blue jacket or yellow sweater. Close-ups recurrently obscure her eyes with her hands, hair, or objects. When angered, her sharp canines flash visibly. Her personality is volatile, erupting into violence against Tadakuni or his friends. This contrasts sharply with bursts of impulsive excitement, like hurling her bag into a tree, followed by embarrassment if noticed. She exhibits curiosity through eavesdropping and nervous habits like twisting a strand of hair. She possesses extraordinary strength: incapacitating Tadakuni with one kick, overpowering multiple opponents single-handedly, and violently shaking a crane game machine unmovable by the main trio combined. Lacrosse gear occasionally accompanies her. Within the narrative, she primarily fuels comedic conflict through aggressive interactions. Appearing in only 15 of 139 manga chapters, her impact resonates through running gags about her underwear and meta-humor like "Tadakuni’s sister’s brother." She ranked 9th in a character popularity poll; "Tadakuni’s sister’s panties" also featured as a separate poll entry.