Jaw is the name given to the parasitic being that inhabits a portion of Mamoru Uda’s body. The creature initially tried to take over Uda’s brain after they met near a seaside cliff, but when its host fell and began to drown, the parasite was forced to abandon its original plan to save both their lives. As a result, Jaw took control of Uda’s lower face, jaw, neck, and upper chest, leaving Uda’s brain and original personality intact. This rare circumstance made the creature one of the few parasites to coexist with a human host without fully consuming their identity.

Jaw’s personality is notably distinct from the cold, predatory nature of many other parasites. It can think logically and take decisive action when necessary, such as shifting Uda’s internal organs to prevent lethal damage during a fight. However, Jaw is also frequently seen smiling and appears generally gregarious, an unusual trait for its species. This emotive quality and its manner of speaking were shaped by watching movies with Uda, who often cries during films, leading Jaw to adopt certain expressions and a rougher dialogue style influenced by what they watched. Despite its often cheerful demeanor, Jaw does not fully understand or respect human emotional sensitivity, sometimes referring to Uda’s tendency to cry as a uniquely human trait that it finds confusing. It can also be rude and insulting toward its host, a tendency that becomes more pronounced as the series progresses. The being puts little importance on names, initially being referred to simply as Parasite by Uda until the term became widely known for monsters, at which point Uda gave it the name Jaw based on its location on his body.

Jaw’s primary motivation is survival, which is inherently tied to the well-being of its host, Uda. The two form an unusual and mostly cooperative partnership built on mutual need rather than deep affection. Jaw recognizes the advantage of working with other beings in similar circumstances and is readily willing to cooperate with Shinichi Izumi and his parasite partner, Migi. This alliance proves valuable when facing hostile parasites, including the one that took over Shinichi’s mother. Jaw and Uda work alongside Shinichi in that battle, with Jaw’s quick thinking and ability to reorganize Uda’s organs allowing them to survive a fatal blow and ultimately help decapitate the enemy. Later, Jaw and Uda assist Shinichi again by helping to capture a private investigator who had been stalking him. Jaw communicates effectively with other parasites using signals and can connect with Migi when in close proximity.

In terms of abilities, Jaw possesses the standard powers of a parasite but with notable adaptations due to its unusual placement on the host body. It can reshape its own tissue, specifically the lower half of Uda’s face and neck, into hardened, weaponized forms for combat. It can also detect and emit the signals that parasites use to identify each other. Because it occupies Uda’s throat and chest, Jaw can stretch out Uda’s mouth to act as a snorkel, allowing him to breathe underwater, and can shift the positions of Uda’s organs to prevent a stab wound from being instantly fatal. However, this placement also creates significant disadvantages. When Jaw stretches out or transforms, it drags Uda forward and causes mild pain. Since they share the mouth area, only one of them can fully control it at a time, forcing Uda to breathe only through his nose, which becomes difficult during intense battles. Unlike many parasites, Jaw has no hair, which allows it and Uda to easily avoid a public method of detection that involved pulling on people’s hair. Over the course of the story, Jaw grows more accustomed to working with its host and allies, learning to adapt its strategies in dangerous situations despite the inherent awkwardness of its position on Uda’s body.