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 villains. The Hero Association ranks all of its members by a Class and a ranking within that class. 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 character, Saitama (サイタマ), is a bald-headed 25-year-old man who is bored of fighting because he is effortlessly able to defeat enemies with a single punch.Vol. 1 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 when he defeated the powerful supervillain Crablante that attempted to kill a child with his buttocks on his chin. Ch. 2 Saitama says he became a hero "for the fun of it".Ch. 5 His abilities mainly consist of physical abilities magnified to an immeasurable degree: strength is the true power of Saitama, with speed, stamina, and durability being mere side powers. He attributes this to a daily training regimen (100 push-ups, 100 sit-ups, 100 squats, a 10 km run, eating healthy food, and not using the air conditioner or heater to reinforce his mental fortitude).(He also mentions training even when his body was wounded and made strange cracking sounds). However, he seems to become stronger when he pushes himself to his limits (three years of that transformed him into what he is today)Ch. 11 Early in the series, he picks up a disciple and roommate Genos.Vol. 1

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.Web ch. 16-17, Ch. 16 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 Speed-o’-Sound Sonic and destroying a meteor similar to the meteor that made the dinosaurs go extinct with a single punch. As of the current arc of the webcomic by ONE, he is A-Class rank 39. He does not mind letting others have the recognition though, as demonstrated in his actions following the fight against the Deep Sea King. In the Dark Matter Thieves' 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, before he could use his Roaring Star Cannon attack. Through various circumstances, he forms friendships with Mumen Rider, Bang, King, Genos, and Blizzard. His superhero name in the association is Bald Cape (ハゲマント, Hagemanto, Anime: Caped Baldy).Ch. 45

Saitama is voiced by Makoto Furukawa in the Japanese version[1] and by Max Mittelman in the English dub.[2][3]

Genos (ジェノス, Jenosu) is a 19-year-old cyborg who becomes Saitama's disciple after bearing witness to Saitama's easy victory over Mosquito Girl.Web ch. 6-7, Ch. 6-7 Four years prior to the start of the storyline, his family was killed by a rampaging cyborg known as Mad Cyborg. Genos was nearly killed in the attack and his body was destroyed so, to keep him alive to stop the Mad Cyborg for justice, a scientist named Dr. Kuseno took Genos in and modified his body to make him a cyborg with destructive capabilities. However, Genos remains steadfastly convinced that justice demanded vengeance against and destruction of the cyborg.Web ch. 7, Ch. 7 His perfect score on the entrance exam for the Hero Association places him in S-Class, but he still considers Saitama to be his master and goes with him whenever he is not summoned to other missions.Web ch. 16-17, Vol. 3-4 Saitama often has to save him, as Genos' headstrong personality causes him to jump into action without thinking. He lives with Saitama after offering a sizable down payment for his share of the rent.Ch. 18 His superhero name in the Hero Association is Demon Cyborg (鬼サイボーグ, Oni Saibōgu)Ch. 45