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.

Bang
The 3rd rank S tier hero Silver Fang became aware of just how powerful Saitama well and truly was during the Dragon level crisis of the Meteor threatening Z City. So much so that he venerated the new blood as being superior to him as a fighter. Bang was so impressed with the mysterious individuals incredible might, he would often try to coax him & Genos into participating in his dojo as students under the flowing water crushed rock style. Repetitively tempting Saitama with a plethora of choice foods and the like from time to time. Silver Fang withholds so much respect & praise that he internally monologued he would hold nothing against him should the Hero decide to quite after the hazing scam by abominable heroes Tanktop Black Hole & Tiger after the threat had passed.

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 realizes Saitama was holding back and it wasn't a fair fight at all.

Blast
Before the fight between Saitama and Cosmic Fear Mode Garou, Blast initially didn't know what to think of the former when he first saw him. Although concerned for his wellbeing due to the lethality of the divine power Awakened Garou now possessed. He never thought much of him due to how menacing the adversary was, given the tense straights of the situation at hand. This all changed in a hurry when an incensed Saitama unleashed his power and the adversary emulated him to match. Blast immediately sensed the energies emanating from their colliding fists and was astounded at the incredible strength he'd witnessed while porting them into space.

Armored Gorilla
After Beast King's defeat by Saitama, Armored Gorilla was aware of the hero's strength. Later, inside the House of Takoyaki 🐙, when Dr. Genus was talking about Saitama removing his limiter to Zombieman, Armored Gorilla was listening to him likely knowing who the scientist was referring to.

Genus
After listening to Saitama's training regiment and witnessing him defeat Carnage Kabuto in a single punch, Genus became aware of the Caped Baldy's immeasurable strength. When talking to Zombieman, the scientist even theorised that Saitama has removed his limiter.

Garou
The hero hunter was largely unaware of the eponymous Caped Baldy's presence throughout his escapade as the Human Monster. Even despite the numerous times they'd encountered one another; at the local bazar, during his dine & dashing and again when he tried to hunt king. His memories always blacked out but Garou still withheld some inkling of his remembrance in the back of his mind afterward. It wasn't until after he'd achieved his Awakened state, as he'd defeated Sage Centipede, did he come to remember who Saitama is and realize just how badly he messed up in completely forgetting about the ultimate unknow factor in his plans. Seeking to draw out and replicate the full might of Saitama's power, Awakened Garou in his Cosmic Fear Mode saw just how powerful his new enemy had become. Commenting that the opposition to his advance as absolute evil was infinitely strong and soon coming to see he could grow ever stronger still.

Overgrown Rover
Saitama and Rover happened to meet by chance when the former accidentally stumbled upon the Monster Association Headquarters by chance while seeking to complain about the noise they caused. The Dragon Level mysterious being flat out attacked him on sight but was quickly delt with by the hero using a simple normal punch. When after regaining consciousness just as Saitama was about to confront the Monster King. He instead fled for his life the very second Orochi ordered it to attack Saitama once more. Clearly acknowledging that the intruder was more than Rover could handle.

Orochi
After witnessing Saitama's strength and the pressure emanating from him, Orochi realized that he was responsible for the death of the two Cadres Elder Centipede and Gouketsu, and the spreading of rumors about the "Ghost Town Horror" in Z-City. Seeing Saitama as the perfect sacrifice, Orochi decided to take him to an ancient mural underground, and after seeing Saitama casually using the lava as a hot tub Orochi decided to use his full power from the very start, gathering energy from the core of the Earth and launching his Gaia Cannon, only to be easily repelled by Saitama's Serious Squirt Gun and then being mortally wounded by a single punch.

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 the few characters who could immediately tell Saitama was powerful before fighting him, as he quickly backed away in fear after sensing he would have been killed immediately if he tried to attack him. As anger got the better of him though, he began to doubt Saitama and, after turning Carnage Mode and losing all reason, no longer feared him.

Bakuzan
After being defeated by Saitama in the Super Fight Tournament, Bakuzan thought that Saitama used some cheap trick to win the match, as he did not remember how the fight had occurred. However, despite being so easily defeated earlier, he assumed that his new monster form should be able to kill Saitama, only to be defeated with a single punch again moments later.

Jinzuren
Jinruzen was one of the first staff members of the hero association to become aware of the abnormal strength that Saitama possessed when he broke all records in the fitness test, in his conversation with Sneck he described Saitama as if God was residing on his body

Phoenix Man
While Phoenix man was trying to convince Child Emperor to become a monster, Saitama broke into his Spiritual Space by punching it, making the villain confused about how Saitama entered there, at first Phoenix thought that maybe Saitama was just wearing a costume like him and Child Emperor but soon after he realized that it wasn't the same thing. He then began to feel strong pressure from Saitama and quickly decided to send them back to the real world, realizing that he would be in trouble if he tried to do anything to Saitama.

Tatsumaki
Tatsumaki discovers Saitama's true power when they fought in the Psychic Sisters Arc. Despite her dislike towards the A-Class hero, she still came to acknowledge his strength, and when she trapped him underground she still admitted that this probably won't kill him. After their fight she admitted that he was "kinda strong".