Humans

Humans (人間, Ningen) are the dominant race in the world, as they outnumber all other races.

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

Human size vary according to its ethnicity and age. Humans possess skin color which ranges in variety from the darkest brown to the lightest hues. Certain body parts, mostly facial structures of humans vary a lot from ethnicity and age.

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 in an unknown period of 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 creating 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 is its intelligence and numbers. They vastly outnumber the Mysterious Beings and possess advanced technology which allowed them to multiply across the planet. However, most of the weapons they use prove to be useless against stronger Mysterious Beings. Humans acquired super powers through experiments, talent and hard work. Other humans were born with powers, like Green. Many of these humans are members of the Hero Association.

Humans defend themselves from many of the stronger Mysterious Beings with the heroes from the Hero Association. Mysterious Beings ranked as Wolf and Tiger are still dealt with using regular weapons.

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.