Saitama (サイタマ Saitama) is the main protagonist of OnePunch-Man, and the most powerful hero alive. Having apparently trained himself to superhuman condition, Saitama faces an existential crisis as he is now too powerful to gain any thrill from his heroic deeds.
He is registered with the Heroes Association as a C-Class Superhero and is tasked to defend Z-City against Mysterious Beings.
For a superhero, Saitama is rather laid back. Because even the mightiest foes pose no challenge to him, he doesn't take his hero work very seriously. In spite of this, he is constantly searching for an opponent that can challenge him, since his superhero work is beginning to bore him because it's too easy. The combination of his attitude, unstoppable strength, and distinctively simple and 'unimpresive' appearance often cause his battles to become anticlimactic. Saitama will usually allow his opponents to rant about their motives and power up into their strongest forms before suddenly and nonchalantly obliterating them with one punch.

Imperturbable and like an unmovable pivot." - Tite Kubo[1]

Genryūsai Shigekuni Yamamoto (山本元柳斎 重國, Yamamoto Genryūsai Shigekuni) was the captain of the 1st Division in the Gotei 13 and also the Captain-Commander (Head-Captain in the English Dub) of the whole Gotei 13. His lieutenant was Chōjirō Sasakibe.

Saitama