A towering warrior characterized by eyes with blackened sclera and crimson hair long enough to reach his waist. Born as the strongest, he has never encountered an opponent capable of making him fight at full power, and thus he constantly wears a bored expression. He pays no mind to those who do not interest him, having killed an incubus who was bothering Brunhilde and obstructing his path as if swatting a common fly. He wears greave-like boots and a loincloth-style wrap, donning a robe when outside of combat. In ancient times, Asgard, the realm of the gods, was on the brink of destruction when it was suddenly attacked by 66 Frost Giants. While the guards and knights were helpless and the residents were being slaughtered, Thor alone was bored. After slaying one of the dull giants with his divine hammer, Mjolnir, and muttering about its weakness, he proceeded to annihilate the remaining giants single-handedly. Even the physical might of the giants, which easily crushed everyone else, failed to make Thor exert his full strength. It seemed he was destined to spend his immortal life in boredom—until he met Lu Bu in Ragnarok, the final struggle between gods and humans.

Three years before the start of the story, while struggling with his job hunt, he saved a child with a cleft chin from the monster Crablante, which inspired him to become the hero he dreamed of being as a child. For the next three years, he devoted himself to intense training and monster hunting, pushing himself so hard that all his hair fell out, but gaining invincible strength in return. He continued his hero work as a hobby for a long time, but after Speed-o'-Sound Sonic told him he had never heard of him, he realized his lack of fame. Upon hearing about the Hero Association from his disciple, Genos, he took the exam with him and became a professional hero. The hero name assigned to him by the Association is Caped Baldy. Naturally, Saitama, who is sensitive about his lack of hair, hates this name and wishes to change it. Fans of One Punch Man often refer to him as "Sensei," a title adopted from Genos. He usually wears hoodies or T-shirts with logos like "OPPAI." He actually has a decent wardrobe and isn't uninterested in fashion, though his sense of style seems slightly off from the average person. When active as a hero, he wears a yellow suit with red gloves and a white cape. According to an OVA, this hero suit is a memento given to him by a tailor he was close with back when he still had hair. Before obtaining the suit, he fought in a blue tracksuit. The color scheme of the hero suit is actually the reverse of a certain famous hero's colors. Though his bald head makes it hard to tell, he has handsome features. Even when facing monsters, he remains mundane, worrying about trash day or supermarket sales. This is a testament to his overwhelming strength—he has the luxury to worry about such things in any situation. In battle, he is ruthless enough to end a fight with one punch even while an opponent is introducing themselves, though he was genuinely put off when Genos incinerated the House of Evolution's hideout immediately upon arrival, noting that the enemy probably had preparations and things to say. While he often seems indifferent, he holds his own convictions and ideals of what a hero should be. Like in the battle against the Deep Sea King, he sometimes takes the blame to preserve the dignity of other heroes. However, he also has a petty side, such as making Fubuki pay for his meal when he forgot his wallet or taking his frustration out on a controller after losing a game to King. In the original and manga versions, he even committed a dine-and-dash during the chaos of Garou's appearance. While he doesn't crave being pampered, he does have a desire for fame and recognition; his motivation for becoming a pro hero was his lament over his lack of popularity. He considers eating three square meals a day part of his training, though he says a single banana for breakfast is fine. While he eats normally, he once gave up easily on a mega-portion spicy udon challenge (which Genos, being a cyborg, finished effortlessly), suggesting he isn't a particularly big eater. Since he hasn't climbed the ranks quickly after becoming a pro, he lives a frugal life. He hasn't touched the money Genos brought when moving in, so he should have savings, but he remains a penny-pincher who gets visibly shaken if he misses a supermarket sale. Despite this, he seems to like the affordable conveyor belt sushi chain "Nezumi Sushi" and goes there with Genos occasionally. To Saitama, being a hero is a "serious hobby" he does for himself; he doesn't fall into despair if his activities have no impact on the world. On the other hand, because he believes an ideal hero is someone who stands up to great evil and wins by a hair's breadth using their full power, he sometimes feels frustrated by ending fights in one hit and gets excited when facing a potentially strong foe. However, he also performs casual acts of kindness, showing he doesn't think fighting evil is the only part of being a hero. He doesn't kill monsters indiscriminately; he spares those who surrender, domesticated dogs, or beings that aren't causing harm.