Saitama is From One Punch Man.Three years before he became a hero,When He was Job Hunting.He Ran into a monster Trying to Kill a Kid,However He was able to somehow kill thr monster.After this he Wanted to be a hero and So he did 100 SitUps, 100 PushUps,100 Sqauts,and Ran 10 km everyday(Somehow breaking his"Limiter"in exchange for his hair,Tho he don't know this)After he met a young hero name Genos later and Join the hero association, becoming A-class.He lives in Z city, which is monster infested