Saitama is the main character. He is a bald - headed hero who can defeat any opponent with a single punch. He's a bit of an underdog in the sense that his power is so great that he's bored with fighting most of the time.
Genos is also a main character. He is a cyborg who becomes Saitama's disciple. He is very powerful but not nearly as strong as Saitama. He is constantly seeking ways to improve his strength and learn from Saitama.
The main character is Saitama. Genos is also important as he is Saitama's disciple. King is another significant character, known for being wrongly perceived as a very powerful hero.
The main character is Saitama. He's the one - punch man. Genos is also very important. He's Saitama's disciple. King is another notable character, known for being wrongly thought of as a very powerful hero. And there's Tatsumaki, a powerful esper hero.
Saitama is the most prominent character in 'One Punch Man Hero Story'. He's the one with the amazing strength of defeating enemies with just one punch. Genos, a cyborg, is another main character. He admires Saitama and becomes his disciple. There are also other characters like King, who has a reputation of being a very powerful hero but in reality, he just has a knack for being in the right place at the right time. And there are heroes from the hero association like Tatsumaki, a very powerful esper.
The main character is King. But other characters from the One Punch Man universe may also make appearances, like Saitama who is often associated with King due to the misunderstandings regarding King's 'heroic deeds' which are actually Saitama's most of the time.