Monster King Orochi

The Japanese manga series One-Punch Man contains a number of fictional characters created by One and illustrated by Yusuke Murata. The series follows a superhero named Saitama and his disciple Genos who join the Hero Association so they can be recognized as such when they fight various monsters and supervillains. The Hero Association ranks all of its members by a Class and a ranking within that class. The following characters listed are ones noted by the author in the manga profiles, ones that were highlighted in the anime character list, and ones that recur over several story arcs.

The title protagonist, , is a bald-headed 25-year-old man who is bored of fighting because he is able to effortlessly defeat enemies with a single punch. He lives in an apartment in City Z. Three years prior to the start of the series, when he still had hair, Saitama was job hunting and lacking a sense of purpose in life when he defeats the monster , who attempted to kill a child with a butt-chin. Saitama says he became a hero "for the fun of it". His abilities primarily consist of immense physical prowess, demonstrating incredible strength, speed, and durability, as well as the capacity for further growth when pushed to his limits. He attributes his powers to following a daily training regimen for 3 years, consisting of 100 push-ups, 100 sit-ups, 100 squats, a 10 km run, eating healthy food, and abstaining from usage of an air conditioner or heater in order to reinforce his mental fortitude. Besides this training, Saitama is otherwise ignorant as to the origin of his power. Early in the series, he picks up a disciple and roommate, .

Though he had broken all the physical records in the Hero entrance exam by huge margins, Saitama enters the Hero Association with a score of 71, giving him one of the lowest ranks in the Hero Association. This is due to his miserable performance on the mental exam. He soon goes up the ranks by performing many deeds, although many of his efforts are not recognized because of collateral damage or higher-tiered superheroes receiving more credit. These include defeating the raging ninja supervillain/assassin known as (who became his unlikely rival) and destroying a meteor similar to Chicxulub, each with a single punch. As of the current arc of the webcomic by ONE, while promoted to A-Class rank 39, he also becomes a leader of Hero Name Victim Association, a group of heroes who are discontent by their current hero names as Saitama, with an aim to demand Hero Association to give them proper hero names during their protests. Regardless of his rank, he does not mind giving others the credit, as demonstrated in his actions following the fight against the . In the ' story arc, he is unfazed by the telekinesis of a powerful esper, further displaying his resistance. This arc was also one of the only times he has used his "serious punch", defeating the world-conquering leader of the aliens, Lord Boros. It was also revealed that Saitama's power can continue to grow limitless as demonstrated during his fight with Cosmic Garo, with the latter leaving him behind on the power scaling through every blows exchanged. Through various circumstances, he forms friendships with , , , , and . His superhero name in the association is .

Saitama is voiced by Makoto Furukawa in the original Japanese version and by Max Mittelman in the English dub.