Grace, also known by her alias Nitta, is a character introduced as a formidable presence from Yuki's past. She is a female assassin and an informant who arrived in Japan specifically in pursuit of Yuki, having been one of the few individuals with knowledge of Yuki's history as an assassin known as Xue, the White Wolf, from the Dragon's Nine organization. Her personal goal is unconventional for an assassin: to spend her later years in comfort on the island of Santorini, attended by men, and she sought to raise her reputation and fund this retirement by targeting Yuki's life.

Initially, Grace presents a direct and dangerous threat. She appears at Hitoyoshi's home, takes him hostage, and challenges Yuki directly, intending to kill her former comrade for prestige. However, she is defeated by Yuki in combat. Following this loss, Grace makes a promise not to make another attempt on Yuki's life. Rather than disappearing, she adopts a new role that allows her to continue observing the situation. She uses her skills in disguise to secure a position as a physical education teacher at Hitoyoshi's school, Ogiso High, operating under the alias Nitta.

Grace's personality is notably carefree and driven by curiosity. She is not characterized by a brooding or vengeful nature after her defeat. Instead, her primary motivation shifts to a fascination with the circumstances of her former colleague. She is deeply intrigued by the fact that Yuki, once the strongest assassin, has chosen to serve as a maid for a completely ordinary high school student like Hitoyoshi Yokoya. This curiosity is the main reason she takes on the teacher role, positioning herself as an outside observer of the relationship between Yuki and Hitoyoshi. Her interest in this dynamic extends to Hitoyoshi himself, and she often engages with him in a provocative manner, with borderline flirtatious or harassing physical contact, which consistently provokes a jealous and retaliatory response from Yuki.

In the story, Grace serves as a significant catalyst and a recurring observer. Her initial attack forces Yuki's past into the open, directly confronting the peaceful life Yuki is trying to build and leading Hitoyoshi to learn the truth about her assassin background. Her return as the teacher Nitta creates a source of ongoing tension and comedic conflict, as she represents Yuki's previous world while promising not to interfere with her new one. Her role develops from a primary antagonist into a mischievous, curious figure who watches the relationship evolve, often testing its boundaries for her own amusement. Her notable abilities lie not only in combat, as she is a formidable fighter, but more specifically in her skills of disguise and eavesdropping, areas in which she herself admits to being more proficient than in direct fighting. These talents are essential to her infiltration of the school and her role as a silent observer. Her personal indulgences include ramen, cigarettes, and alcohol.