Characters aware of Saitama's power

It's as if God himself resides in the body of Saitama. Despite being the most powerful being in the series, Saitama's power is mostly unacknowledged, especially before registering as a professional hero for the Hero Association, due to him not claiming his achievements.

However, few characters are aware of it by sensing Saitama's potential or witnessing his abilities first-hand. Even so, some still reserve doubt on his true power, whether because of their pride, prejudice, or skepticism towards his comparatively low hero rank and simple training regimen.

Genos
Genos is among the few who are aware of the full potential of Saitama, as he witnessed him attain effortless victories in multiple fights against formidable opponents. Genos glanced at Saitama's strength for the first time when he saved him from defeat against Mosquito Girl, leaving him so impressed to start an apprenticeship under him. Genos starts accepting Saitama's inexplicable power in their sparring match which leaves him wondering if he will ever reach his master's level. Despite that, Genos still thought Saitama would need help to defeat Gouketsu.

Boros
Boros was one of two characters able who could immediately tell Saitama was powerful before fighting him. He was able to sense no limit to his energy. Though as he brags about his retentive abilities, he underestimates Saitama for his human body as it will "grow tired over time." At the end of their fight he finally realises Saitama was holding back and it wasn't a fair fight at all.

Speed-o'-Sound Sonic
After meeting and being defeated by Saitama for the first time, Speed-o'-Sound Sonic swore eternal rivalry and vowed to defeat him. Recognizing his power, Sonic stops working and focuses on training. Despite having already lost to Saitama in three one-sided battles, Sonic still trains in the hope to surpass him one day and even accepts monsterification as a way to achieve that goal.

Carnage Kabuto
Carnage Kabuto was one of two characters able who could immediately tell Saitama was powerful before fighting him. Quickly backing away in fear as he sensed that he would have immediately been killed if attacking. Though he starts to doubt Saitama when getting angry and turning Carnage mode.