Yukimachi Tsukishiro holds the title of Tortoise Knight within the Beast Knights. She possesses long silver hair styled in pigtails and matching silver eyes, typically seen wearing a school uniform while maintaining a characteristically relaxed expression with her eyes closed. In later years, she wears her hair loose.

Her personality is outwardly laid-back, showing initial indifference to the Beast Knights' conflict. Yet she reveals formidable inner strength in moments of fear, inspiring others through her courage and resilience. Her mentor Akitani observes a consistent blindness to her own weaknesses.

As a Beast Knight, Yukimachi wields Domain Control, manifesting as telekinetic power. Her abilities dramatically increase during combination attacks with Subaru Hoshikawa, the Rooster Knight. When joining hands, they create the Invincible Shield—an impervious barrier protecting the entire group and reflecting assaults. Their second technique, the Strongest Spear, unleashes a devastating energy blast. These cooperative powers originate from their shared training under Akitani, refining his inherited techniques.

Yukimachi shares her deepest bond with constant companion Subaru Hoshikawa. This friendship drives her pivotal decision to use her contractual Beast Knight wish to secure Subaru's future happiness. Following the main conflict's resolution, her relationship with Taiyou Akane, the Owl Knight, develops into a romantic involvement during the subsequent ten-year period.