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".

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 has killed countless people, 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 A-City and causes a large explosion in the middle of the city, then throws multiple energy balls, destroying much of it in several locations. Lightning Max and Smile Man were sent after Vaccine Man, but are beaten. He then notices and prepares to kill a crying school child, but she is saved at the last second by Saitama. 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. Credit for defeating him ultimately went to King.

Abilities and Powers
Vaccine Man is a creature of great strength, being able to destroy a large part of a big city, as well as defeat 2 A-Class heroes by himself, though he was quickly defeated by Saitama.

Supernatural Abilities
Energy Balls (エネルギー弾, Enerugī-dan): He can shoot energy balls which cause large-scale explosions, large enough to destroy cities.

Flight (飛行能力, Hikō nōryoku): 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 (変身, Henshin): 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

 * He is the first Dragon level threat to appear in the series.
 * Vaccine Man's normal form bears a strong resemblance to the character Piccolo from the popular manga series Dragon Ball. Coincidentally, the character happened to share the same voice actor, Christopher Sabat.
 * His Japanese voice actor is Ryusei Nakao, who also voiced Frieza in Dragon Ball Z.
 * Much like Demon King Piccolo from original Dragon Ball was born out of humanity's evil influence, Vaccine Man was born out of humanity's pollution.
 * Vaccine Man happens to be a parody of the character Baikinman, the primary antagonist of Anpanman. Given that One Punch Man is a reference to Anpanman, Vaccine Man shares the same Japanese voice actor as Baikinman, Ryusei Nakao.
 * Murata stated that the energy balls he uses are the same as Choze and Homeless Emperor.