Mamoru Uda is a human who, like Shinichi Izumi, survived a parasite encounter with his own consciousness intact. In his case, the parasite did not reach his brain but instead lodged in the lower part of his head, around his jaw and neck area. He works as an employee at a hotel and lives a relatively ordinary life before his transformation. Physically, Uda is very tall, has an average build with a slightly overweight frame, and keeps his black hair neatly combed. His parasite, which originally has no name but is later referred to as Jaw or Joe, learned human language by watching movies and dramas. Unlike many other parasites that are blunt and exclusive, Joe develops a cheerful and sociable personality with a brash, crude sense of humor. He sometimes takes control of Uda's mouth, distorting it into odd shapes and forcing Uda to say strange or inappropriate things.

Uda himself is a peaceful, easygoing man who is kind-hearted and empathetic. He is emotionally fragile and tends to cry easily, especially when under pressure. Despite his nervous disposition, he possesses a strong sense of justice and can act with surprising bravery when protecting others. His warmth and vulnerability set him apart from the more stoic or cold characters in the series. His motivations are not driven by grand ambition; rather, he prefers to avoid conflict with other parasites and simply wishes to live quietly. He becomes friends with Shinichi and Migi after recognizing their shared condition, and he feels deep compassion for Shinichi's harrowing experiences.

Uda's role in the story is that of a supporting ally. Although he prefers to avoid danger, circumstances push him into combat situations alongside Shinichi. His key relationship is with his own parasite, Joe. The two coexist distinctively, with Joe often acting as the more assertive and talkative half of the pair. Their bond is functional and cooperative, allowing them to act together when needed. Uda also forms a meaningful friendship with Shinichi, built on mutual understanding of what it means to share a body with a parasite. His development is subtle: he starts as a timid, emotionally overwhelmed man but grows into a reliable ally who willingly helps Shinichi face threats.

Notable abilities stem from the unique placement of his parasite. Because Joe is located in the lower part of Uda's head, he can extend his parasitic tissue downward into Uda's chest, protecting his heart, which is a common target in parasite duels. This defensive capability gives Uda an edge in prolonged fights. However, because Uda is slightly overweight and his parasite is situated near the jaw, physical exertion tends to leave him out of breath quickly. Additionally, Uda possesses the ability to hold his breath for extended periods, a skill that proves useful in specific confrontations. In combat, Joe is the primary fighter, using his parasitic extensions offensively while Uda's body endures the strain. Together, they form an effective but unconventional team.