In the world of Nisemonogatari, Tsubasa Hanekawa stands as a figure of exceptional intellect and quiet kindness, yet one whose internal world is marked by profound complexity. She is a third-year high school student and the class representative at Naoetsu Private High School, a role she embodies with such dedication that her classmate Koyomi Araragi describes her as the class representative of all class representatives. Her initial appearance in the broader story, which carries into her role in Nisemonogatari, is characterized by her signature braids and glasses, though in this season, she undergoes a notable external change by cutting her hair short and appearing without her glasses. Her catchphrase, "I don't know everything, I just know what I know," reflects a humility that stands in stark contrast to her encyclopedic knowledge.

To understand Hanekawa in Nisemonogatari requires looking at the foundation laid in previous events. Prior to the series, she was possessed by a bakeneko, a supernatural oddity, during Golden Week due to intense stress stemming from her troubled family situation. This event gave rise to Black Hanekawa, a manifestation of her repressed negative emotions. Her family life is a source of this deep-seated stress; she lives with her stepfather and stepmother, and her biological mother died by suicide shortly after her birth, leading to a complex and emotionally distant home environment. While the possession was resolved with supernatural help at the cost of some memories, the underlying issues were not eliminated, making her a person constantly struggling against her own suppressed feelings to maintain a facade of perfection.

In Nisemonogatari, Hanekawa's personality continues to be defined by the dichotomy between her public image and her private reality. On the surface, she is the ideal student and a fundamentally kind and humble person who is always willing to help others. She is highly principled and does not hesitate to intervene when she perceives an injustice, as seen when she objects to Araragi's harsh treatment of his younger sisters, Karen and Tsukihi, reminding him of his own principles against violence. However, this perfection is a carefully constructed front that hides her deep-seated emotional instability and her method of coping by severing inconvenient memories and emotions, a subconscious act known as Heart Severance. Her motivations are largely driven by a desire to remain correct and upright, managing the stress of her home life and her unacknowledged feelings for Araragi, which had previously been the catalyst for the reappearance of her oddity.

Her role in the story of Nisemonogatari, while not the primary focus of a dedicated arc, is significant as a pillar of support and a source of stark honesty for Araragi. She appears at the Araragi household to mediate during his conflicts with his sisters, demonstrating her role as a moral compass and a figure of calm authority. A key aspect of her relationships in this season is her dynamic with the two most important people in her life. Her relationship with Araragi is one of deep mutual respect and friendship, built on the events of the past. He respects her more than anyone else. However, this closeness creates tension with Araragi's girlfriend, Hitagi Senjougahara. In a notable display of her hidden power, Hanekawa has effectively warned Senjougahara to treat Araragi well, implying she could take him away, a threat that the usually dominant Senjougahara takes seriously, revealing that others are well aware of the depth of Hanekawa's feelings and capability. Her interactions with the Araragi sisters also show her as an authoritative yet caring figure who is not afraid to correct their behavior.

The development Hanekawa undergoes throughout the Monogatari series is a long-term process of self-acceptance, with seeds of change present in Nisemonogatari. Her physical transformation by cutting her hair is symbolic of a willingness to change, a small but conscious step away from her prior fixed image. The full resolution to her internal conflict comes later, in the Tsubasa Tiger arc, where she is forced to confront the tiger spirit Kako, a manifestation of her own jealousy, and ultimately accepts Black Hanekawa as an integral part of herself. This acceptance results in a physical change where a stripe of white appears in her hair, symbolizing the integration of her light and dark sides. Her journey is thus not about being cured of her oddity, but about acknowledging and absorbing her flaws and negative emotions to become a more complete person.

Regarding her notable abilities, Hanekawa possesses two distinct categories of power. As a human, she is a genius-level intellect, consistently achieving the top exam scores in Japan and possessing a broad and deep well of knowledge that she can recall instantly and apply effectively. As the host to the supernatural oddity, her alternate form, Black Hanekawa, possesses superhuman physical characteristics, including small building level attack potency and supersonic speed. Black Hanekawa can also drain the energy and vitality of those she touches, which can heal her wounds and causes temporary memory loss in her victims. This form is essentially a powerful, agile, and silent combatant driven by Hanekawa's most primitive instincts and repressed emotions.