Keiji Ueji is one of the escaped convicts from Abashiri Prison, and his appearance in the city of Sapporo serves as the primary catalyst for the events of the final season, drawing all major factions to the city in pursuit of the Ainu gold tattooed on his skin. His most distinctive physical features are the numerous tattoos covering his face and neck, which he has explained were self-administered. He has long, dark hair often styled into braids and typically wears a hat with a pom-pom on top, a floral-patterned coat, striped pants, white gloves, and geta sandals on his feet over long socks. A notable part of his attire is a white sheet of cloth or paper worn as a mask over his face with two holes for the eyes, tied around his head with a thin string. He usually carries a box tied around his neck with a rope, which contains a puppet he uses in his performances or to startle others. His teeth appear to have been filed down to give him an unnatural look, and he has dark circles under his eyes, though these were not present in his childhood.

Ueji's personality is defined by a bizarre and macabre obsession: his favorite thing in the world is to see the disappointed expression on another person's face. He is a trickster who derives an abnormal amount of joy from pranking others and lying to them, solely for the sake of eliciting a look of disappointment, at which he will burst into laughter. Fellow convict Boutarou the Pirate describes him as a weird guy for this very reason. Despite his penchant for jokes and pranks, he harbors a deeply negative and pessimistic worldview, explaining to a child that the world is full of nothing but disappointments. This cynical outlook is rooted in his traumatic childhood.

The motivations behind Ueji's twisted behavior are explored through his background. As a child, he was subjected to immense pressure from his father, a celebrated soldier who fought in the Battle of Hakodate, and from everyone around him who expected the boy to live up to his father's heroic legacy. He was constantly compared to his father and told to study hard, to take pride in his lineage, and to become a great soldier. This oppressive environment caused him to act out at the Military Preparatory Academy, which led to a confrontation with his father, who warned him that if he was expelled, he would no longer be welcome at home, adding, "Aren't I always telling you? Don't disappoint me". The breaking point came when his father gave away his beloved dog, Jirou. Convinced his father had killed and buried the animal, a distraught Ueji dug up the garden looking for the corpse and broke down. In his distress, he tattooed the kanji for "dog" on his own forehead with a needle and showed his father. When his father realized the mark was permanent and could not be wiped away, he became deeply disturbed, and Ueji laughed at the look of disappointment on his face. This event seems to have transformed him, shifting his tears of sadness into joyous laughter at the disappointment of others.

In the story, Ueji operates as a candy peddler, using his guise to lure children, whom he then abducts and is implied to kill. He is also active in Sapporo, where he commits a series of murders that lead the public and the various factions to believe a serial killer in the style of Jack the Ripper is on the loose, adding another layer of chaos to the hunt for the gold. His key relationships are largely transactional or antagonistic. He is pursued by the Seventh Division of the Japanese Army under Lieutenant Tokushirou Tsurumi, by the faction of former Shogunate loyalist Toshizou Hijikata, and by Saichi Sugimoto's group. He demonstrates a prior awareness of the escape artist Yoshitake Shiraishi, recognizing him immediately during their encounter, though Shiraishi does not seem to have known of Ueji. While Ueji appears as a primary target in the final season, his development is largely revealed through flashbacks to his childhood and youth, which explain the origins of his pathological need to create disappointment in others as a coping mechanism for a life that was, to him, nothing but a series of let-downs.

Regarding notable abilities, Ueji possesses no particular combat prowess like some other convicts, but his skills lie in deception and psychological manipulation. He is a proficient liar and a trickster, using his unsettling appearance and unexpected actions, such as giving a child a piece of coal instead of candy, to create the reactions he craves. His most defining ability, however, is his pain tolerance and dedication to his own bizarre aesthetic, as he personally tattooed his own face and neck. His value to the other characters is purely informational, as the map to the Ainu gold is tattooed onto the skin of the convicts, making him a living piece of the treasure map that every faction is desperate to acquire.