Tsubasa Hanekawa is Koyomi Araragi's classmate and close friend at Naoetsu Private High School, admired as an overachieving model student. Her family history involves significant instability: her biological mother became pregnant at 17, married a workaholic stepfather, and committed suicide shortly after childbirth. This stepfather later died from overwork, leading to multiple remarriages. Hanekawa now lives with a stepfather and stepmother biologically unrelated to her, sharing distant and abusive relationships. She sleeps in the hallway without her own room and cooks separately, reflecting the fractured household.

Externally, Hanekawa appears soft-spoken, kind, and academically exceptional, consistently achieving top grades as class representative. She frequently downplays her knowledge with the phrase, "I don’t know everything, I just know what I know," especially around Koyomi to make him happy. Her calm demeanor enables patient interactions, even with challenging individuals like Mayoi Hachikuji. However, this persona masks profound internal instability. Her upbringing led her to suppress all negative emotions and selfish desires, striving to be a "pure white" human acting solely by societal norms. This repression creates intense psychological stress; she internalizes abuse and neglect without objection, believing she deserves her suffering. Her inability to express her true self or seek help deepens her fragility.

Her first meeting with Koyomi occurs during spring break when a wind gust lifts her skirt, revealing her underwear and sparking their conversation. She actively pursues his friendship despite his initial reluctance. Following Koyomi's transformation into a vampire, she investigates rumors about Kiss-Shot, provides him with clothing and advice, and witnesses his battles, sustaining life-threatening injuries. Koyomi heals one injury using his vampire blood, leaving traces of Kiss-Shot's blood dormant within her. She later prevents Koyomi's suicide by revealing the method to regain his humanity—killing Kiss-Shot—and navigates tense romantic moments with him, including an instance where he nearly touches her breasts before choosing a back massage. Meme Oshino observes an unsettling flawlessness in her behavior during this time.

Stress from her unrequited love for Koyomi and abusive home life culminates during Golden Week. After burying a dead cat without emotional engagement, she consciously accepts possession by the stress-draining oddity "Sawarineko," creating her violent alter ego "Black Hanekawa." This entity attacks townspeople at night, leading to confrontations involving Koyomi and Meme. Though temporarily resolved, her unresolved emotions later manifest as "Kako," a flaming tiger symbolizing envy, after witnessing her step-parents reconcile without her. This aberration threatens to burn homes, reflecting Hanekawa's anguish over lacking a place to belong.

Hanekawa evolves significantly through these events. She cuts her hair and switches to contact lenses as symbols of growth, while her hair develops silvery-white tiger-like stripes post-"Tsubasa Tiger," which she dyes black daily. Crucially, she confronts and accepts her repressed emotions and alter egos, abandoning her "perfect" facade. This allows her to confess to and be authentically rejected by Koyomi, acknowledging her pain without self-deception. Her personality shifts toward greater self-assertion, evident when confronting Ougi Oshino or vampire hunters. After graduation, she travels globally, engaging in hazardous NGO work—aiding refugees, removing landmines, and digging wells—earning the moniker "Japanese Joan of Arc." She later transitions to war mediation, erasing national borders to end conflicts, a role that renders her internationally wanted.