Yamato Kai lives in Akatsuki Special Zone's B1 district, home to illegal residents, sharing the space with his younger brother Minato Kai. He stands 182 cm tall with orange hair and eyes. His typical attire includes a forest green jacket, black trousers, red Timberland boots, and a necklace. Born July 20th, his zodiac sign is Cancer.

Four years before the main events, the Akatsuki Rebellion claimed his parents. During this conflict, Eiji Arashiba saved Yamato's life by transferring the Cerberus tattoo to him. This act made Yamato a Scard—an individual bearing a tattoo that seals divine powers. Cerberus grants him heightened sensitivity to the scent of murder and immunity to conventional bullets. Even before receiving this power, he possessed considerable physical strength and impressive combat agility, traits noted by Eiji.

Initially wielding a standard six-shot magnum revolver, he later acquired dual revolvers once owned by Kazuma Arashiba. Outwardly stoic and reserved, Yamato speaks little and maintains a cold demeanor. This masks a powerful sense of justice; he reacts instantly to calls for help and frequently aids B1 residents without prompting. This drive stems directly from his deep admiration and gratitude towards Eiji, whom he views as the "Hero of Akatsuki."

He struggles with practical domestic tasks, relying on Minato for household management. Yamato also exhibits a remarkably low tolerance for alcohol, becoming easily intoxicated even from its scent. His relationship with Minato is protective yet complicated by Minato's inability to assist him directly, reflecting their shared history of loss and survival. The trauma of the rebellion and Eiji's rescue profoundly shaped Yamato, fueling his commitment to protect others within Akatsuki's lawless environment.