Komachi Hikigaya is the younger sister of Hachiman Hikigaya. With both parents frequently absent due to demanding corporate careers, Komachi and Hachiman manage a semi-independent household where she often takes charge of meals and domestic duties. She presents as an average-height middle school student with medium-length black hair featuring a distinctive ahoge, similar to her brother's. Her expressive eyes contrast Hachiman's "dead fish eyes." She typically wears her middle school uniform before later adopting Sobu High School's female attire. Cheerful, sociable, and occasionally forgetful, Komachi's personality sharply contrasts Hachiman's cynicism. Despite this, she possesses notable intelligence and social intuition. She slyly engineers situations between Hachiman and female acquaintances like Yukino Yukinoshita or Yui Yuigahama, maneuvers he usually recognizes. Her "Big Three" attributes—cuteness, prettiness, and good looks—grant her school popularity, undiminished by her association with her isolated brother. Beneath her easygoing nature lies perceptiveness and strategic thinking akin to Hachiman's. Genuinely kind, she can turn seriously cold when offended, evidenced by instances of refusing to speak to her father or Hachiman until receiving proper apologies. She selectively engages socially based on situational advantages, reflecting awareness of both social and reclusive lifestyles. Her relationship with Hachiman is foundational. She deeply cares for him, relying on his academic help while reciprocating through domestic care. She actively counters his social isolation and "rotten personality," often through teasing or manipulation. Komachi understands his underlying kindness despite his abrasive exterior and mediates between him and others. She recognizes his "social suicides" even without witnessing them directly. Her fabricated childhood story about running away, created to help Saki Kawasaki understand Hachiman's concern, illustrates her nuanced grasp of his protective nature. Komachi maintains positive connections with Hachiman's acquaintances. She exchanges contact information with Service Club members and assists with activities like promotional magazines and summer camps. She shares mutual respect with Yukino Yukinoshita, who envies their sibling bond, and has a relaxed friendship with Yui Yuigahama, recognizing Yui's feelings for Hachiman. Her friendship with Taishi Kawasaki includes studying together and mutual encouragement to attend Sobu High School, with hints Taishi may harbor feelings. She interacts comfortably with others like Saika Totsuka (initially mistaking him for a girl) and Shizuka Hiratsuka, who uses Komachi to monitor Hachiman. Her practical abilities include accomplished cooking skills; she regularly prepares meals for herself and Hachiman with precise knowledge of his preferences. Academically capable despite Hachiman's teasing about being an "idiot," she often borrows his old projects for reference. Komachi navigates challenges stemming from Hachiman's social reputation, particularly during middle school when his embarrassing nickname "Otagaya" complicated her social standing. She employed strategic maneuvering to secure her student council position by publicly declaring her ambition, effectively deterring competitors concerned about social repercussions.