Haruki Sagae, a student in the clandestine assassin group "10th Year Class Black," balances a calm, carefree demeanor with steely resolve. Often seen casually snacking on pocky or shrugging off discussions of mortality, her relaxed facade masks a fierce dedication to safeguarding her adoptive family—nine siblings she supports through her deadly vocation. Light-skinned with piercing gold eyes, she sports long red hair tied in a ponytail, barrettes pinning her bangs, and a purple hair tie around her right wrist alongside layered bracelets. Her school uniform—a beige jacket, white shirt, red plaid bow tie, and skirt paired with brown shoes—contrasts with her combat-ready studded fingerless gloves.

Raised in a sprawling household, Haruki turned to assassination to ensure her family’s survival, channeling her first earnings into essentials and a single indulgence: pink nail polish. She joined Class Black for a promised reward to secure their future, meticulously strategizing her mission while blending socially with classmates. Her plan to collapse a gym roof during an assassination attempt, sacrificing herself to guarantee her family’s compensation, ultimately failed. Graciously accepting defeat after severe injuries, she was expelled and shifted to construction work, juggling labor with college aspirations.

In the manga continuity, she navigates a relationship with former classmate Isuke Inukai, who prods her to return home. Haruki’s past reveals a duality of nihilistic acceptance and relentless determination, choosing family over self even in dire stakes. Though her laid-back exterior and calculated manipulations hinted at strategic cunning, her schemes faltered, leaving her to rebuild a life anchored in resilience and quiet perseverance.