Keiya Utō is a central figure in his story, serving as the underboss of the Senagaki yakuza group. At twenty-six years old, with a birthday on December twenty-second, he has taken on the critical responsibility of being the primary caretaker and guardian for Isaku Senagaki, the group leader's teenage granddaughter.

In his role, Keiya is defined by a powerful sense of duty and a deeply ingrained protective instinct. He volunteered to become Isaku's bodyguard and is notably physically strong, capable of defending her when necessary. However, his protection extends far beyond physical threats. Convinced that adolescent boys and romance are inherently dangerous for Isaku, he takes extreme measures to oversee her daily life. This includes using connections to enroll as a high school student at the same school she attends, where only the principal knows his true identity and he is introduced as her childhood friend.

Keiya's personality is a mix of serious earnestness and an eccentric, overbearing nature. He possesses a strong awareness of his position as her guardian, often treating the fifteen-year-old Isaku as a much younger child, which leads to bouts of overprotection that sometimes go too far. He can be oblivious to sarcasm and is described as somewhat socially inept in certain situations, yet he consistently demonstrates a kind and composed demeanor. This contrast is most visible when he poses as a student, where he easily displays a natural charisma and becomes popular among his peers, much to Isaku's surprise. A recurring behavior is his immediate and harsh treatment of any male student who tries to get close to Isaku, as he firmly believes men are dangerous.

Keiya's motivations are rooted in a promise to protect Isaku at all costs and a sincere desire for her happiness above all else. This dedication is not just professional but deeply personal, stemming from their shared history. When a five-year-old Isaku first came to live with her grandfather after losing her parents, she was initially wary of the then-younger Keiya. However, his patience and comforting words helped her find solace, and from that moment, their bond was sealed. This foundational event shaped his entire approach to her, blurring the lines between duty and genuine affection. He has described his own feelings as wanting to be a mother, father, brother, and lover to her, embodying whatever she needs him to be. This depth of feeling is symbolized by a tattoo on his back representing his lifelong responsibility to her.

His role in the story is directly tied to his relationship with Isaku. He is her constant companion, acting as both a watchful guardian and, over time, a romantic partner. While he begins by seeing her as a child to be protected, the narrative follows the evolution of their relationship as Isaku matures and her own romantic feelings for him become undeniable, a sentiment he eventually reciprocates, leading them to become a couple. Besides his combat ability, Keiya is remarkably skilled in all household chores, making him a proficient caretaker in every aspect of domestic life. His physical appearance is notable; he is a tall, handsome young man with soft black hair that features an undercut, three piercings on his ear, and sharp, calm slate-blue eyes.