One Punch Man's History
For a superhero, Saitama is thought to be quite indifferent. Even the mightiest foes pose no challenge to him, so he doesn't take his hero work very seriously and beats monsters like swatting insects. Since his superhero work has begun to bore him, he is constantly searching for an opponent that can challenge him. The lack of any such opponents has led him to suffer from an existential crisis, and he claims that his ability to feel any and all emotions has dulled considerably. He stated that he just became a hero because it was his dream and for self-satisfaction and before becoming one, used to suffer from anxiety issues. Despite his boredom, Saitama does not ignore crime, as in his own words (having displayed a lack of understanding at the disaster level system), "If the heroes run and hide, who will stay and fight?".
The combination of his attitude, unstoppable strength, and distinctively simple and 'unimpressive' appearance often cause his battles to become anticlimactic. Saitama will usually allow his opponents rant about their motives and power up into their strongest forms before obliterating them with a single punch. However if they talk too much he will either interrupt them or just punch them.
A running gag about Saitama is his inability to remember people's faces and names, seen when he forgot Sonic's name and could not remember who Tanktop Tiger was. Because of this, he always ends up pronouncing their names incorrectly.
As of some special chapter during his time participating a costume contest, appearance doesn't matter to him, the only thing matters to him are his abilities and his desire to be a the super hero he's been training for. That being said, he does care about his cleanliness, as the only thing he ever really complains about is how he gets dirt on his clothes or blood on his gloves. Though he doesn't care about appearances, he has shown dislike towards his hero name, Caped Baldy. Thus even after receiving it, he always refers to himself as Saitama.
Saitama has been shown to be very humble, as he purposefully let the masses turn against him, for the sake of the defeated heroes to be given credit for their efforts; claiming that they had weakened the Sea King before his arrival. He did the same for the police in the Special Chapter, dressing as one of them and killed a monster, though he would had gain much fame had he revealed who he actually was. He also didn't mind that King took credit for all of his achievements.
Another surprising thing about Saitama is that he is surprisingly tactful and insightful, being well able to discern situations and character traits from the people and environment around him fairly easily. Having picked up a number of hints to Sonic's own persona on their first meeting and when two tank top heroes jealously plotted to ruin him after the meteor event that there were irate and self-serving hero types like them, as well as discerning Fubuki's forced offer of submission or destruction just before his crackdown on how the hero world really works than how it is currently run.
Saitama has been shown to show remorse to his enemies before, particularly ones who can give him an actual fight and don't go down in one hit. One such example of this being Boros, to whom, in order to give an exciting fight to (seeing as he could relate to having similar amounts of power), though in the end, still couldn't draw out Saitama's full power.
Saitama does not actually care if people insult him and he usually ignores them and says that they need a hobby; the only insult that could at the very least get a what could be called reaction out of him is when someone mentions his bald head. He also shows a measure of concern when it comes to bargains, as seen when he fought Carnage Kabuto, when he was more shocked upon the realization that he was going to miss a bargain sale, rather than his opponent, who had undergone a transformation and was whacking him around. Despite his general indifference to most things, Saitama was genuinely shocked to when he realized that no one actually knew of him, despite his achievements (although this could be attributed to the fact that he never stuck around after saving the day).
Saitama shows impressive restrain with his power, considering that with his physical ability, he is capable of splitting the sky, or jumping from the moon to earth easily. He manages to beat nearly all of his opponents with a single punch, though occasionally causes more devastation than necessary. While he has no problem killing monsters, Saitama has yet to be seen killing another human, and uses only enough strength to knock them out.
Little is known about Saitama's past, but it is known that he lives in his apartment by himself. As a child, according to Saitama, he used to watch superhero shows many times, and wishes that he wants to be like those heroes too someday.
Middle School Year
In his second school day of his freshman year in middle school, Saitama is seen reading a health and PE manual, but did not hear from the other students about what happened to the freshman who called himself "The Bicycle Commuter" yesterday, after two third-year bullies beat him up pretty badly.
After school on a rainy day at 4 o'clock, Saitama is watching a news about the ongoing monster incident in Z-City, news of which happened nine months ago. Saitama drifted off to sleep but forgot to do his homework. As a result, he is called by his homeroom teacher to the teacher's room, despite suggesting that he could do both days' homework that night.
While going to the teacher's room, he is interrupted by the same two third year bullies from yesterday, and threatened to hand over his wallet, but Saitama told them he doesn't have one and prepares to fight them instead. However, Saitama is still too weak to defend himself from the bullies and his 200 yen was stolen by them. Shortly after, Piggy Bancon attacks the bullies and takes Saitama's stolen money from them, Saitama soon heard that one bully's reason in threatening the freshmen to steal their money was to feed his little brother his lunches and Saitama goes chasing after the pig monster to get his money back, but due to his lack of strength, is unable to retrieve his 200 yen. After the monster is destroyed by the police and special unit forces, Saitama decides to return to school and see his homeroom teacher.
Saitama is then scolded by his homeroom teacher, while trying to tell what happened to himself back before going to the teacher's room, though the teacher thinks Saitama is bluffing. While going home, Saitama becomes very disappointed in himself for being weak and losing many times, and starts to think about himself and his future.
Adult Year. First Hero Act
More than three years prior to the start of the story line, Saitama was an unemployed man on a job hunt. One day, after yet another failed interview, a nihilistic-feeling Saitama was wandering the streets and encountered Crabrante. Saitama's total apathy toward the situation impresses Crabrante, who claims to share a similar outlook, prompting him to spare Saitama and move on. Crabrante tells him that his real prey is a child with a cleft chin anyway.
Walking on, Saitama spots the child Crabrante was talking about. He decides that he doesn't want anything to do with the matter but after Crabrante suddenly arrives, Saitama impulsively saves the child from Crabrante's attack. He urges the child to run away and demands to know why Crabrante wants the boy dead. While Crabrante explains the prank the boy pulled on him, Saitama chuckles and notes aloud how the monster resembles one from an anime Saitama watched in his youth. This enrages Crabrante, who pummels Saitama even after the latter resolves to live out his childhood dream of being a superhero by fighting back. As Crabrante turns from the apparently defeated Saitama to finish the cleft-chinned boy, Saitama lassos one of his eye stalks with the loop of his removed tie and tears it out. The monster is defeated and the child saved. Unknown to him, the child confronted his grandfather for the creation of the Hero Association in Saitama's honor for saving his grandchild from the monster's crime.
Resolving after this encounter to become a mighty superhero, Saitama started a daily training schedule which consisted of 100 push-ups, 100 sit-ups and 100 squats along with running 10 km every single day. He ate three meals a day, but only a banana as breakfast and also didn't use the AC, to strengthen his mental resistance. These daily activities were extremely tough and caused his body excruciating pain; muscle aches and internal bleeding. However, Saitama didn't give up and continued his training day after day.
Personification of a Light Pull Cord Attack
On Saitama's 300th day of training, his body had already felt the effect of working out too much. However But still, Saitama has shown signs of improvement, such as wrecking a car after saving a boy from an accident. At that time, the Personification Of A Light Pull Cord attacked the streets, looking for an opponent that would satisfy him.
The Personification punches cars and buildings as he keeps searching. To test the fruits of his labor, Saitama challenged the monster but was sent flying after a single punch. Filled with determination to overcome his limits, the monster punches Saitama again and is knocked down. However, Saitama quickly got up, spat out a tooth that had been bothering him because of a cavity.
Saitama then defeated the monster with a single punch before moving on with his workout.
At some point in the first year and a half of Saitama's training, the Octopus Claw Man attacked an unspecified group of people. Saitama arrived and defeated the Octopus Claw Man, though sustained multiple injuries from the fight. After the fight, he consoled one flailing civilian who had suffered three scratches above his left eye from the mysterious being. When questioned about his injuries, Saitama shrugged it off, simply smiling at the thought that there was a special sale that day.
Magicicada Larva Attack
Some time between when Saitama started training and started to lose his hair, Z-City experienced a Mysterious Being attack by the 170,000-Year Magicicada Larva, prompting the citizenry to evacuate to the state-of-the-art Shelter Nr. 7. The three low-ranking heroes sent to defeat the Magicicada were defeated, but survived and managed to find some straggling evacuees on their way back to the shelter, among them being Saitama.
When Saitama discovers that he has been led to a shelter, he became upset, stating that he thought that following the heroes would lead him to the Mysterious Being. Saitama attempts to leave so he can find the Mysterious Being, but is restrained by the citizens, thinking he's become suicidal. The guards attempt to explain that the doors can't be opened while the disaster is still ongoing, but reassure the public that the shelter is able to withstand any sort of attack. While she praises the structure's perfection, Saitama confronts one guard again to ask where the bathroom is, only for her to reply that the 'perfect' shelter was nevertheless designed and built without bathrooms.
Saitama slowly becomes more and more distressed, as he doesn't know if his bladder can hold until the situation ends. After overhearing that two A-Class heroes were defeated by the beast, and that it will take three hours for a Hero Association squadron to arrive, Saitama can't take it anymore. He walks to a wall of the shelter and breaks a hole in it with a single punch before the stunned onlookers, and runs off stating that he's going to the convenience store. Just as he is about to reach the store to use the bathroom, the 170,000 Year Magicicada Larva appears in front of him, which he promptly kills.
One and a half years after he started training, he had lost his hair and become strong. He continued his training for another year and a half before starting his (initially amateur) superhero career.