B

B is a nameless parasite who confronts Shinichi. He is a minor antagonist.

Appearance
B appears to be a middle-aged man of average height with shaggy blond hair, and is seen wearing a purple vest jacket. In the manga, B appears to have blond and black hair. His face is younger, as well as much more serious and pointed. He is also more muscular and tough looking, giving him the appearance of a punk. He has black paint or markings around his eyes and wears a black sleeveless shirt and jeans.

Personality
Like many parasites, B believed humans to be the lower species. Though Migi had failed to take complete control over his host, he held no ill will towards him and instead offered Migi multiple chances to join him in hopes of prolonging their lives. He is also shocked when Migi kills him to protect his own human host.

He is somewhat philosophical, comparing human eating habits to their reactions to the mincemeat murders. He also advises Migi to read more when Migi questions if he can really transfer hosts.

Plot
B is first seen sensing Migi from a distance, and pursues them when Shinichi flees. After catching up to them, he is surprised that Migi failed to take control of the brain. He offers Migi the choice of joining him, of which Migi is distrustful. He then cuts off his own right hand, again pressuring Migi to join forces with him, claiming that together they can share a host and live at least 140 years. Due to blood loss and Migi's indecision, he attempts to attack Shinichi in hopes of forcing Migi to relocate. However, Migi immediately retailates and slices him in three. B is distraught at seeing his destroyed body and dies while reaching out towards it.

B's death throes may have made Shinichi reluctant to personally kill A.

Abilities & Powers
Being a parasite, B can shapeshift from the head up and and harden his cells for offensive or defensive purposes, as well as detect other parasite signals and automatically emit his own. He can also maximize the host body's physical potential and disregard pain.

He had knowledge of parasite physiology and how merging with another parasite's host would work. It's unknown if he learned this by guesswork, trial and error, or if he was involved with Reiko's experiments in some way.

Weaknesses
Like all parasites, he requires a host body with functioning organs to survive, and would die if he were separated from the body or it was destroyed. Fire can cause his parasite cells to "freak out", making him lose control. He is also susceptible to poisons or caustic chemicals, which would either kill parasite cells on contact or make his host body fail.

He was too hasty in assuming Migi would join him, cutting off a hand before Migi even had made a decision. This led to his death when he attempted to force Migi's decision by attacking Shinichi.

Trivia

 * His name is displayed as B on the official anime website. He has no relation to the "Killer B" joked about by students during Hideo Shimada's rampage.
 * He remained entirely nameless for the manga.