Saitama is One Punch Man's titular character. His world is constantly under attack from monsters that originate on Earth, and a visit from beings with space faring abilities also is not out of the question. While still a salaryman and out on a job hunt, he is confronted by one of these monsters, and is drawn into a fight that sets him on the path to join the Hero Association.
The Hero Association was founded by Agoni, a multi-millionaire whose grandson is saved by a passing stranger from an untimely demise. This organization helps manage heroes located in different cities, moving them around as needed. They are also responsible for identifying and tracking new heroes with the National Superhero Registry.
Saitama is a superhero just for the fun of it, looking to find an opponent that will actually challenge him. His training regimen consists of 100 push-ups, 100 squats, 100 sit-ups and a 10 kilometer run. The training regimen may seem rather normal at first glance, but Saitama swears that this is all he has done to achieve his strength, which is on another level when compared to any of the other characters. This immense strength will eventually earn him an unwanted disciple, Genos.
Any hero who wants to join the association starts by taking the Super Hero Entrance Exam. Saitama gets a perfect score on the fitness portion, but bombs the written portion. This leads to him being placed into the C-Class portion of heroes. The association divides the heroes into four different classes: C-Class, B-Class, A-Class and S-Class.
C-Class heroes are responsible for low level tasks such as stopping purse snatchers or helping get balloons out of trees for children, and these tasks must be completed on a weekly basis or the hero will be dropped from the registry. On the other end of the spectrum are the S-Class heroes, of which Genos belongs to, a group of the most elite heroes who are called upon to take on the most challenging of tasks such as fighting off strong monsters.
There are only 17 heroes who occupy the ranks of the S-Class. They are all formidable fighters capable of great feats, but none have shown the capacity to compete with the likes of Saitama. Bang (Silver Fang), the 3rd highest ranked hero, alludes to this fact after seeing Saitama save City-Z from being annihilated from a meteor.
This brings us to Blast, who is the number 1 ranked hero in the world. Unfortunately Blast has yet to make an appearance, and does not bother to come around during the massive fight at the end of the first season. Not much is known about Blast, although it seems as if he is reluctant to participate in Hero Association meetings. While just about every hero can be ruled out as Saitama's equal, Blast cannot be overlooked. While this character has been a no show so far, there has to be a reason for his top ranking.
While viewers are kept in the dark in regards to the highest ranked hero there is still plenty of action and destruction to be enjoyed courtesy of Saitama, the guy who just does it for fun!