Humans

Humans (人間, Ningen) are the dominant living beings in the world.

Overview
Humans, or Homo sapiens (ホモサピエンス, Homosapiensu), are a race of sapient beings that inhabit the world of One-Punch Man. They are the dominant and most abundant species on the planet.

A human's size varies according to its ethnicity and age. Humans possess skin color which ranges in variety from dark brown to light hues. Certain body parts, mostly facial structures of humans, vary depending on ethnicity and age, and genetics.

A running gag in the series is that a lot of humans are unable to accept Saitama's immense strength despite witnessing him in battle. For example, they call him a fraud after he defeats the Sea King despite him defeating the monster in one punch, as well as blowing the rain away. Another time is when Saitama battles Suiryu in the Super Fight Tournament, the security staff didn't want to intervene when Suiryu was fighting Saitama, yet they were willing to try to pin Saitama down and capture him, not even thinking of how Saitama was unfazed by Suiryu's brutal attacks and defeated him in the first place. Furthermore, during the hero costume competition, most of the audience states that the monster Saitama effortlessly kills is fragile or even self-destruct not noting the fact that many heroes even A-class cannot defeat it. All these factors indicate that the majority of humans suffer from Sunnydale Syndrome.

Furthermore, many humans care more about the reputation and appearance of a person without taking their true power into consideration. Only a handful of people like Bang, Mumen Rider, Sonic, Fubuki, King and Suiryu are perceptive enough to determine the true power of a person. Despite the situation, it is shown that some people or heroes are willing to accept Saitama's tremendous power, either through witnessing or losing to him in combat. For example, Sweet Mask who used to only care about appearances was willing to praise Saitama after seeing him defeat Garou. The police chief also calls Saitama a hero after he brings back the people's hope in the police force and Sneck admits that Saitama is stronger than him after he defeated him in a single blow.

Plot
Humans appeared on the world at an unknown period in time. They were the dominant species in the world prior to 15 years ago, when a race of creatures labelled Mysterious Beings appeared and started wreaking havoc and killing humans. At first they could be easily dealt with, but with time Mysterious Beings grew stronger in power to the point where regular weapons were useless against them. In order to fight the Mysterious Beings, the Hero Association was created.

Abilities & Powers
Humanity's main power lies in its intelligence and numbers. They vastly outnumber the Mysterious Beings and possess advanced technology which allows them to multiply across the planet. However, most of the weapons they use prove to be useless against stronger Mysterious Beings.

Some humans acquire superhuman abilities through experimentation, talent, or hard work. Other humans, like Green, are born with powers. Many of these humans are members of the Hero Association.

Heroes from the Hero Association are humanity's first line of defense against Mysterious Beings. Humans also defend themselves from Mysterious Beings using weaponry.

Some humans are born with "brilliance" which gives them unique abilities. Dr. Genus considers himself to be one those humans.

Humans also possess the ability to transform into Unusual Type monsters under certain circumstances.

Trivia

 * It's hinted by Dr. Genus that Saitama's powers were granted by breaking his human limits.