Tsukasa Yuzaki, born Tsukasa Tsukuyomi, possesses an origin deeply entwined with Japanese folklore. Her story begins roughly 1400 years ago within the legend of Kaguya-hime, where a desperate servant saved his dying daughter by giving her the Elixir of Hourai meant for the emperor. This act granted Tsukasa immortality, freezing her physical age at sixteen and bestowing extended life, regeneration from fatal wounds, and resistance to illness. Her endless existence demanded centuries of periodic identity changes. Over this vast time, Tsukasa amassed profound historical knowledge, evidenced by her ability to decipher archaic texts, familiarity with ancient landmarks like Kyoto and Nara, and skill with traditional artifacts such as katanas; she often recounted historical events as personal memories. She maintained a unique bond with the moon, symbolized by her ownership of a moon rock from her grand-aunt Tokiko and recurring lunar imagery during pivotal moments. Tsukasa first met Nasa Yuzaki on a snowy night, saving him from an oncoming truck. Though critically injured herself, she insisted the driver take Nasa for help before vanishing. Nasa later found her, confessed his feelings, and accepted her marriage proposal as the condition for dating. Two years passed before she reappeared at his apartment with a marriage registration form, commencing their union. Initially serene and emotionally contained, Tsukasa gradually revealed vulnerabilities within her marriage. She displayed jealousy when Nasa complimented others, embarrassment at his affectionate proclamations, and a longing for mutual admiration, explicitly asking him to call her "cute." Her personality merged traditional domestic skills—cooking, cleaning, managing the home—with modern passions for gaming, films, and pop culture. Key aspects included stoicism towards physical pain and a pragmatic outlook, yet she treasured sentimental acts like holding hands and preserving mementos such as their wedding plant. Her relationship with Nasa involved mutual adaptation: she fostered his emotional openness while managing her concealed immortality. Tsukasa largely kept her past hidden, though she acknowledged the solitude of eternal life and her quest to regain mortality. Her development centered on embracing marital intimacy and confronting her secretive existence, with unresolved tensions surrounding her immortal nature and its future impact.