Saitama is a Superhero who save people for fun
Once a young man leading a fairly mundane life on the job circuit, a chance meeting with a crab-like mutant threatening a young boy reawakened Saitama's childhood dream of being a superhero. After saving the child from the mutant, Saitama embarked upon a brutal training regime that cost him all of his hair but granted him a superhuman constitution. He has, however, become a victim of his own success as he is now so strong that any opponent he faces is defeated with one punch, which leaves his superheroic pursuits deeply unsatisfying.
Saitama was created by a mangaka who uses the pseudonym "ONE" for his webcomic One-Punch Man in 2009. In 2012, ONE was approached by Yusuke Murata to consider a collaboration between the two, and the One-Punch Man remake was the result.
Saitama is quite a short, plain-looking man in his mid-twenties with no-hair. He often appears quite slightly-built, but when he is shown with less clothing it is apparently that his body is actually rather well-toned. While working as a hero, he wears an all-yellow body suit with either a white or red cape and red gloves and boots.
Saitama is usually a casual, somewhat lackadaisical and down-to-Earth sort of person. He can however be annoyed by fairly minor things, and seems to avoid fuss. He is quite comfortable in his own skin, however(though he is somewhat touchy about his hair), and seeks no-ones approval, what he looks for from superhero-ing is his own satisfaction. While he doesn't always seem particularly serious about it, he does show concern for others, and won't let others come to harm if he can stop it.
So far, Saitama hasn't had recurring contact with anyone in the series apart from Genos. He has found the young cyborg's keenness and seriousness irritating at times, especially when he first met him, but they seem to have developed a genuine friendship since, and Saitama tends to look out for his self-appointed disciple when he's in trouble.
Saitama has phenomenal physical prowess, particularly his strength. He has destroyed walls, smashed superhuman beings into pieces and even destroyed a gigantic meteor, all with a single punch in each case. This strength also allows him to make superhuman leaps. He is likewise too durable to be harmed by virtually any form of physical attack displayed so far, and was fast enough to keep up with and out-manoeuvre the ninja Speed-of-Sound Sonic, a ninja famed for his speed.