Vaccine Man

Vaccine Man (ワクチンマン Wakuchin Man) is a Mysterious Being born due to the massive pollution released across the Earth by human beings. According to him, he was created as an incarnation of nature's wrath, meant to wipe out humanity and thus effectively vaccinate the earth against future infections. He is the first enemy faced by Saitama shown in OnePunch-Man, and the first shown to fall victim to Saitama's (in)famous fist.

Appearance
Vaccine Man is a hairless humanoid, about the size of a large man, with two rounded antennae on his head. He has a very muscular body and wears no clothing. He is capable of transforming part or all of his body into a larger, more powerful form somewhat resembling a dinosaur and armed with vicious claws and fangs.

Personality
Although Vaccine Man is a mass murderer, he considers himself to be acting from a position of righteousness. In his mind, the violent destruction of humanity is necessary and just when weighed against the entirety of the rest of nature. Vaccine Man seems to become enraged at the very thought of humanity's crimes against nature.

Introduction Arc
Vaccine Man appears in the peaceful Z-City, identified as a Demon Level "Mysterious Being". He causes a large explosion in the middle of the city, destroying a great deal of it. He then throws multiple energy balls which cause enormous explosions in multiple locations. Just as he is about to kill a weeping school child, Saitama suddenly swoops in and takes her to safety. Vaccine Man inquires about Saitama's identity to which he replies that he is simply an average guy who serves as an average hero. Vaccine Man, offended at Saitama's casual attitude, launches into a diatribe about his name and purpose while transforming into his second form. He is quickly defeated by Saitama in a single punch, which becomes the staple of his heroic deeds for the rest of the series.

Abilities
Vaccine Man is a strong creature. He was able to destroy a large part of a big city by himself, though he was defeated by Saitama later on.

Energy Balls: He can shoot energy balls which cause large-scale explosions, large enough to destroy cities.

Flight: Vaccine Man's supernatural powers also have the extent of flight. He is surrounded by an aura that's similar to his energy balls and takes off several feet in the air.

Transformation: Vaccine Man has the ability to partially or completely trasform his body into a much larger form bearing claws, teeth and visible scales. This is likely a more powerful 'battle-form', as he adopted it just as he was about to attack Saitama. The extent of this form's power is unknown, as Vaccine Man was swiftly killed by Saitama almost immediately after adopting it.

Trivia

 * Vaccine Man's normal form bears a strong resemblance to the character Piccolo from the popular manga series Dragon Ball.