Chizuna Takashiro is the elder sister of Kazuna Takashiro, the protagonist of the story. She is a central figure who has lived with the burden of the Takashiro family's genetic affliction for most of her life, unlike her brother who was kept in the dark about it. Chizuna resides in the family home, having lived there with her father, Shizuna Takashiro, until his suicide a year before the main events of the narrative. Because she remained in that environment, she is intimately familiar with the curse that plagues their bloodline, a condition that manifests as an anemic-like weakness and an overwhelming thirst for human blood that only be sated by drinking it.

Her personality is shaped profoundly by years of isolation and suffering. Chizuna is portrayed as withdrawn, carrying a deep-seated weariness and a somber acceptance of her fate. She is not world-weary to the point of passivity, however; she demonstrates a strong, self-sacrificing will, particularly concerning her younger brother. Having witnessed the toll the curse takes, she is determined to protect Kazuna from descending into the same despair and dependency that she knows so well. Despite her fragile demeanor, she possesses a fierce inner resolve to bear the cross of their lineage, often acting as a guide to Kazuna as he begins to experience his own violent awakenings.

Chizuna's primary motivation throughout the story is to manage the curse for herself and for Kazuna. She regularly takes custom medicine prescribed by Doctor Minase, a family confidant, to stave off her cravings, though she knows these are a temporary measure rather than a cure. Her role in the narrative is that of a catalyst who reveals the family secret to Kazuna and becomes his anchor. She tries to provide him with the support and understanding she never had, hoping they can endure their shared affliction together. However, her motivation is complicated by a deep, tragic loneliness; she and Kazuna are the last of their line, and no one outside the family can truly comprehend their suffering.

The most significant relationship in Chizuna's life is with her brother, Kazuna. Their dynamic is fraught with tension and poignant tragedy. Upon their reunion, she frequently notes the strong physical resemblance Kazuna bears to their deceased father, a connection that leads her to sometimes project memories or feelings onto him. As the story progresses, their shared isolation and mutual dependence foster an intense bond that blurs the lines between familial love and a more romantic, forbidden attachment, as they feel they are the only two people in the world who can understand each other's pain. Another key relationship is with Doctor Minase, who is in love with her. He tends to her medical needs and represents a potential lifeline to the outside world, yet Chizuna remains emotionally distant from him, her focus almost entirely consumed by her own condition and her brother’s well-being.

Throughout the narrative, Chizuna undergoes a development that is less about personal growth and more about a deepening descent into the tragic fate of her family. Initially presented as the sister who has learned to live with the curse, she gradually reveals the immense toll it has taken on her mental and physical health. She grows more desperate to prevent Kazuna from making the same mistakes and suffering as she has, sometimes leading her to make extreme sacrifices. Her arc is one of tragic acceptance, moving from simply surviving to actively trying to break or circumvent the cycle of suffering for Kazuna's sake, even if it means plunging deeper into her own darkness. Physically, she is noted to have a weak constitution and a frail heart, a direct consequence of the affliction she carries. In terms of notable abilities, Chizuna does not possess supernatural powers; her condition is a curse that provides no superhuman strength or vampiric traits beyond the desperate craving for blood, which is portrayed as a debilitating illness rather than a source of power.