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.

Mafia boss in the city of Naples, in Italy, a part of the Sione family. He is known for his cruelty and viciousness. A bom attempt perpetrated by one of those close to him made him a paraplegic. However, with the firs chance he got with the wheelchair that he modified himself, he persued the traitor who'd flied in Ferrari and eliminated him by running over his car.

Fifteen-year-old yakuza leader and strongman. When his father died, he was adopted by his uncle who saw great potential in him. His grip is so strong that if he grabs an opponent's arm, he can cause it to explode from restricting the blood flow. He is also a street fighter and it is unknown if he has trained in martial art styles. He broke Yuri Chakosky's arm, thus ending his boxing career, but when then cross paths again they hold no grudge against each other understanding each other's warrior spirit.

Hayato is Miu's grandfather and the undefeated elder of the Ryōzanpaku Dojo, who is said to have beaten 500 Shaolin masters to near death. He is also known as the 'Invincible Super Man' (無敵超人, mutekichōjin?, lit. 'man with no equals'). His kicks have speeds that are still a blur even when viewing a replay of it at 1/1000 of the normal speed. If necessary, he is able to intimidate the other masters in order to maintain peace. Even using just 0.0002% of his power, he can easily overwhelm both Kenichi and Miu at the same time and rip a tree in half. He was able to defeat a master level opponent in D of D tournament with just one finger. Prior to building the dojo, he and his granddaughter had traveled around the world in his quest for peace. He has 108 special techniques so far and can copy any move an opponent uses against him, like Kenichi's 'Saikyou(Strongest) Combo' and 'Saikyou Combo 2' , Soutenshu and Miu's 'Hyōi Kazekirihane'. He knows Seikūken and is the creator of "Ryūsui (flowing water) Seikūken" which Kenichi learned from him. However, his most used technique seems to be the 'Human Shuriken.