Characters Who Know How Strong Saitama Really Is.
Genos has witnessed Saitamas full power on multiple accounts, be it their training bout in season one or him seeing Saitama defeat countless foes in one punch. Genos strives to be like his Sensei and his drive stems from extreme and utter awe of Saitamas power. However since he is a cyborg he is only as good as his parts or in this case the upgrades that he gets from Dr. Kuseno. Some examples of of enemies Saitama has killed in one punch are, Mosquito Girl, The Deep Sea King.
Dr. Genus was convinced that other than him his species had a long way to go and could only strive for improvement. This was his way of saying that in its current state humanity was worthless. However saitama changed that thought process for him completely. After his ultimate creation or in other words the culmination of the house of evolution, Carnage Kabuto was decimated in one strike from Saitama, Genus finally realized that he was the one that had to change.
Speed of Sound Sonic.
Sonic has held a grudge against Saitama for defeating him ever since their first encounter. However even then sonic realized that saitama was way out of his fighting league. For instance in their second encounter, sonic swings a blade at saitamas face and he bites the blade and shatters it into pieces. This alone should give sonic a comparison of their active power levels. But sonic still perseveres in hopes to one day beat saitama.
Bang, or Silverfang is one of the chosen few to have seen the full brunt of Saitamas power. When a giant meteor was threatening to fall and level City Z, Silverfang and the demon cyborg were the only ones present on the scene and entrusted to take care of it. However before they could take action, Saitama jumped up and shattered the meteor in a single hit. This proved to bang that Saitama was indeed the strongest man he had ever met, which he openly admits in his monologue shortly after.
Mumen Rider is also one of those fortunate people that have seen Saitamas full power. In the incident with the Deep Sea King, Mumen Rider goes at the sea king with all his might but to no avail. But he is taken aback when he sees saitama, a fellow C Class, beat the creature with such extreme ease. Mumen Rider may not be the most powerful hero, but he is definitely one the most if not the most powerful example of the essence of heroism in all the Hero Academy’s roster of heroes.
King is known to be one of the more cowardly heroes from the S-class of the hero academy. However his situation is clearly not his own doing. He was known to simple be at the wrong place at the wrong time. He witnessed Saitamas full power when he and Saitama are attacked by a giant bird that Saitama stops with one hand. He then proceeds to completely destroy the bird with one punch. King witnessed this and realized how truly strong Saitama is.
Blizzard Of Hell Fubuki
Fubuki comes off as one of those characters that judge people based on their appearance or social standing. However her mindset is swiftly corrected as she tries to recruit Saitama to her blizzard group. However he refuses her offer and shows her that there is more to being a hero than ranking and forming groups. She realizes Saitamas power when he renders her efforts to crush his spirit useless.
Suiryu is the most recent addition to the roster of characters that have witnessed Saitamas power. He faced Saitama himself and would have taken full force of one of his hits had Saitama not pulled his punch. However he now knows the power that Saitama holds when he witnessed him destroy first Bakuzan and then Goketsu with a single hit. He was so awestruck that he even asked Saitama to make him his disciple.