Vaccine Man

Vaccine Man (ワクチンマン, Wakuchin Man) was a Mysterious Being supposedly 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 was the first enemy Saitama faced in both the anime and the manga and also the first enemy to be defeated by Saitama with one punch, thus introducing the audience to Saitama's great power and his frustration over it.

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 parts or all of his body into a larger, more powerful form somewhat resembling a dinosaur and is 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 Saga
Vaccine Man appears in Z-City and is identified as a Demon Level "Mysterious Being". He causes a large explosion in the middle of the city and throws multiple energy balls, destroying much of it in several locations. He then notices and prepares to kill a weeping school child, but is prevented from doing so by Saitama, who quickly takes her out of harm's way. Vaccine Man inquires about Saitama's identity to which he replies that he is simply an average guy who serves as an average hero as a hobby. Offended at Saitama's casual attitude, Vaccine Man 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, the staple of the latter's heroic deeds for the rest of the series.

Differences in the Anime
In the anime, Lightning Max and Smile Man are sent after Vaccine Man and beaten before the latter lays waste to a large part of a city and encounters Saitama. Vaccine Man is also killed mid-dialogue by Saitama, as opposed to shortly after announcing his intent to eradicate mankind.

Abilities & Powers
Vaccine Man is creature of great strength, being able to destroy a large part of a big city by himself, though he was quickly defeated by Saitama.

Supernatural Abilities
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.

Physical Abilities
Transformation: Vaccine Man has the ability to partially or completely transform 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. His Battle Form also resembles the appearance of the video game character Diablo, more specifically the version from the game Diablo II.
 * In the anime, Vaccine Man is voiced by Ryusei Nakao, who also voiced Frieza, among other villains from the Dragon Ball series. Interestingly, he also voiced Baikinman, who is the main antagonist of Anpanman, which is the basis for One Punch-Man.
 * Similarly to how Saitama appears to be based off of Anpanman in appearance and through the manga's title, Vaccine Man appears to be based off of Anpanman's main antagonist Baikinman.