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Saitama is a powerful hero who easily defeats all of his opponents with a single punch. He is a thin man at the current age of 25 and is bald, supposedly due from his excessive training. Saitama has always been heroic, saving the innocent from the strange Mysterious Beings that appear in his world; however, he never receives any credit for it. After defeating so many without effort, he feels bored and empty inside, endlessly searching for opponents that will actually pose a challenge to him.
Saitama originally was an ordinary man constantly looking for a job who became depressed after many rejections. One day, Saitama saves a child from a monster. This actions spawns two major consequences: the child's powerful relative creates the Hero Association, and Saitama decides to become a hero like the ones from his childhood. After training every day for three years with an inhuman workout routine of 100 push-ups, 100 sit-ups, 100 squats and 10 km running, he gains superhuman strength and speed but loses his hair. Although Saitama believes his strength is a result of everyday workout, Genos finds it ludicrous.