Karin Yuuki, formerly known as Judas Iscariot and nicknamed Isht Karin Orte, is the immortal designated Number 4 within the organization UQ Holder. Her combat prowess and resilience earned her the title "Saintess of Steel." Originally Judas Iscariot, she betrayed the first person she admired, an act that transformed her into an immortal, a state she considers a curse. This past fuels deep self-loathing and a conviction that she is unforgivable. Centuries later, Yukihime saved her, instilling unwavering loyalty and motivating her membership in UQ Holder. Physically, Karin possesses short black hair, black eyes, and a toned physique. She typically wears a school uniform during missions and a maid outfit otherwise. A distinctive tattoo featuring the roman numeral "XIII" beneath a star with wing-like designs marks her back. She wields a long hammer and a sword carried horizontally across her lower back. Karin maintains a stoic and reserved demeanor, often expressionless. She is strict with UQ Holder members, especially newcomers, offering blunt assessments of their abilities. Her devotion to Yukihime borders on reverence, initially causing intense jealousy towards Touta Konoe due to his close relationship with Yukihime. This jealousy manifests in frequent physical and verbal reprimands against Touta for perceived immaturity or inappropriate actions. Karin harbors a strong aversion to perversion, reacting aggressively to instances involving Touta or others like Xingzi Chao. Despite her harsh exterior, she shows compassion to vulnerable individuals, reassuring an orphaned child worried about immortality or expressing regret over failing to intervene earlier in Sayoko's mental decline. Her self-hatred and view of her immortality as a curse sustain her belief in her unworthiness of redemption. Her character evolves significantly through interactions with Touta. Initial hostility gradually shifts to mutual respect during missions protecting children in the slums or combating threats like the Demolition Man. After a failed Pactio with Yukihime, Karin confides in Touta about her past as Judas Iscariot, leading to an emotional breakdown where she acknowledges him as a friend. They later form a Pactio during a lightning storm in an embrace described as a "lovers' embrace," where she endures pain while gripping him tightly. This deepens their bond, culminating in romantic feelings. After being stranded alone in space for 45 years during a mission, suffering severe emotional torment from isolation and longing for Touta, she confesses her love upon rescue and becomes more intimate with him. Karin participates in pivotal missions, including infiltrating Amano-Mihashira Academy to investigate an immortal killer, where she forcibly alters Kuroumaru's uniform. She is ambushed by Sayoko and buried 1,000 meters underground, requiring later rescue. During the Fighting Fate Arc, she travels to the past with allies to confront Fate Averruncus. Her combat abilities highlight her title "Saintess of Steel"; she effortlessly withstands attacks from foes like the Demolition Man and swiftly counters with her sword. After forming a Pactio with Touta, she aids in critical battles resisting the corrupted Negi Springfield. Her relationships extend beyond Touta and Yukihime. She shares a respectful dynamic with Kuroumaru Tokisaka, encouraging them to choose a female gender to align with Touta. She maintains professional camaraderie with Ikkuu Ameya, valuing his combat skills. Her loyalty to UQ Holder's mission—protecting society's outcasts and policing rogue immortals—underscores her actions.