Fighting games based on anime & manga are often tasked with dealing with power levels between its cast. How can you have a Dragon Ball fighting game where Ultra Instinct Goku exists on the same roster as Hercule? The developers can either establish that some outside force is making everyone equal or just ignore the power difference altogether.
One-Punch Man: A Hero Nobody Knows has the problem of trying to include an all-powerful hero who can defeat anyone in a single punch. Saitama is indestructible and seemingly cannot be defeated, but he's the main character of the franchise and can't be ignored. As such, One-Punch Man: A Hero Nobody Knows has to include Saitama while also keeping the rest of the cast relevant. It does this with a gimmick.
Saitama's Mechanics
One-Punch Man: A Hero Nobody Knows is a 3 versus 3 fighting game. Saitama can be chosen as a playable character and he's just as powerful as he is in the anime/manga, which means he takes no damage from enemy attacks and can defeat anyone with one hit. If two Saitama face each other, then the two can harm each other, but the Saitama who runs out of health first will just run off to the supermarket.
You might think this is would make Saitama the best character in One-Punch Man: A Hero Nobody Knows, but there is a drawback. If the player chooses Saitama for their team, then he won't appear until three minutes have passed. This means that the player has to survive with two characters against the opponent's three. If the two characters are defeated before Saitama arrives, then the player loses the match.
Waiting For One Punch
It's possible to restrict the use of Saitama in the menu for those who want to avoid the mechanics relating to the character. There is also a character called "Dream Saitama" who is wearing pajamas. Dream Saitama is a much weaker version of the character that is on an even footing with the other heroes and villains, for those who still want to be able to play the character without using the Arrival mechanic.
One-Punch Man: A Hero Nobody Knows also has random item drops, some of which can reduce the time it takes for Saitama to arrive, but there is no guarantee that that particular item will appear during the battle.
One-Punch Man: A Hero Nobody Knows is available now for PC, PlayStation 4, and Xbox One.