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
Bang became aware of just how powerful Saitama was during the Giant Meteor incident, having witnessed the Caped Baldy destroy the gigantic rock with a single punch.

Boros
Boros was one of the few 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 did not 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 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.

God
"God" sent Sage Centipede and Evil Ocean Water, two immensely powerful and gigantic Mysterious Beings, to eliminate Saitama, addressing him as "The Fist That Has Turned Against God", showing that the mysterious entity has seen Saitama as a threat. During the fight between Garou and Saitama in the ruined Z-City, it was implied that "God" was watching them battle, as seen when it offered Garou's powers to overcome the Caped Baldy after the Hero Hunter was defeated by the hero. It is unknown exactly when did "God" become aware of Saitama's strength.

Garou
Garou 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 had become a monster, did he come to remember who Saitama is and realize just how badly he messed up in completely forgetting about the ultimate unknown factor in his plans. During their fight, Garou directly witnessed the overwhelming strength Saitama possessed, likening him to the embodiment of unfairness.

King
Saitama saved King from Octopus Claw Man during his first one-and-a-half-year of training, and for a long time, King kept wondering who was that mysterious man that saved him. Saitama would, unknowingly, continue to save King from various monsters, and King took the credit for killing at least five Mysterious Beings that were actually killed by Saitama. Later, when Giant Crow smashed into his apartment, but Saitama saved him again, and since then he holds Saitama in high esteem and considers him an extremely powerful hero.

Dr. Kuseno
Despite never having seen Saitama in action, Kuseno is aware of his strength due to how much Genos has talked about Saitama to him.

Overgrown Rover
Rover happened to meet Saitama by chance when the latter accidentally stumbled upon the Monster Association Headquarters while seeking to complain about the noise they caused. The dog monster flat out attacked him on sight but was quickly dealt with by the hero with a single punch. When after regaining consciousness just as Saitama was about to confront Orochi, Rover instead fled for its life the very moment the Monster King ordered it to kill Saitama, clearly acknowledging that Saitama was more than Rover could possibly 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.

Busho
Initially warry to send him out against a Disaster Level Demon threat. Busho became one of the first HA staff members to purview Saitama's great strength after looking up his hero record on file. Seeking to test for himself just how powerful the new hero really was with the okay from Jinzuren when the two overlooked the former's hero profile. He was one of the many companies exacts assistants whom pushed for Saitama to move up a class after his resounding victory over the Deep Sea King.

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. Even commenting to a sales promotor once that the S-Class hero Genos wasn't the only special talent they scouted during the House of Evolution aftermath.

Fubuki
Fubuki met Saitama in an attempt to get him into joining the Blizzard Group because she worried about her rank, Intrigued by how the hero effortlessly denied her petition, Fubuki starts to attack him will all her strength, but Saitama easily evades her attack and scolds her for going there despising lower ranks, to later protect her from Genos and Sonic’ attack. Fubuki got aware of how strong he actually is, as she saw how he defeated Sonic with his Serious Side Hops. Fubuki, however, is still persistent in her efforts to recruit Saitama, resorting to persuasion or trickery to have him join her group.

Geryuganshoop
Initially seeing him as an unknown intruder onto the Dark Matter Thieves ship, Geryuganshoop found out about Saitama's might when he tried to forcibly remove him from the premise when his psychic powers had no real effect on him. Neither increasing gravity nor tossing rocks and sub-lightspeed had any real effect on him. The ship coordinator met his end when the infiltrator flung one of his own stones back at him. Geryuganshoop died in complete and utter disbelief at his defeat by the former's hands.

Glasses
A former member of the Blizzard Group whom saw firsthand what Saitama was all about while starting out as a charter member. Seeing his display of great strength when he dispatched the Demon level threat Demonic Fan in a single punch inspired him to leave the group and pursue his own lifestyle as a hero, continuing to grow and progress with newfound perseverance thanks to the hero who saved him.

Flashy Flash
Flash mistook Saitama for a monster during their first encounter and the two had a brief scuffle. Flashy Flash tried attacking him with his sword but the latter easily parried it twice, greatly surprising Flashy Flash. Later, Flashy Flash in an attempt of losing Saitama, he rapidly starts running away from him but Saitama manages to keep pace with him, completely shocking the S-Class hero. They soon explore the Monster Association Headquarters together. Flash gives the Caped Baldy the cold shoulder saying he must defend himself and states that he would leave him behind if he cannot keep up. The S-Class hero found Saitama irritating, but after some time spent together lost in the base he got impressed by Saitama's fighting prowess. This gives Flash some respect towards the bald cape-clad hero but he claims that he still has much to learn.

Manako
Saitama first met Manako while wandering in the Monster Association Headquarters, taking Manako hostage to show him the way around. At first Manako tried to find a way to kill him alongside Flashy Flash, but when she was shown to be in danger, Saitama did not hesitate to destroy the monster that was about to attack her. That was when she realized he was going to be too hard to kill and decided to stay with Saitama and Flashy Flash.

Nyan
Nyan happened to be racing by whilst trying to avoid Puri-Puri Prisoner on his way through. He could feel the massive shockwave emitted from Saitama striking down Overgrown Rover and moved to scope out the scene, only to be surprised at how easily Overgrown Rover was defeated. Seeing an opportunity, Nyan moved in to catch Saitama off guard, only to find his sneak attack had no effect on him whatsoever. Fearing for his life, Nyan could intuitively sense he was no match for said unknown hero and fled the Monster Association on a dime. Later on, after having been defeated by Drive Knight, Nyan lamented the poor choices which led to his capture, bringing up there were people out there (referring to Saitama) whom could easily defeat Monster Association executive members to Drive Knight and the newly arrived Genos.

Phoenix Man
While Phoenix man was trying to convince Child Emperor to become a monster, Saitama broke into his Phoenix 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, convincing himself that he would be in trouble if he tried to do anything to Saitama.

Sneck
Sneck was originally unmoved by the revelation of Saitama's ability conveyed to him by Jinzuren after the hero entry examination, not taking kindly to the fact that someone he felt was unworthy of being capable of overtaking him in the ranking system. Sneck opted to rookie crush the newbie, but failed miserably.

Mumen Rider
Mumen Rider became aware of Saitama's strength after he was saved by him from Deep Sea King. Unlike the others whom immediately turned on Saitama when he put on the appearance of a show stealing fraud, Mumen Rider saw through this façade and sent him a letter in thanks for both his rescue and preserving his heroic pride.

Bofoi
Bofoi had been monitoring security footage of the newly renovated fortress City A. Initially perplexed at how easily a no name hero managed to dismantle his robotic enforcers singlehandedly, despite their capability of dispatching Demon Level threat mysterious beings. He steadily grew intrigued at the prospect of an unknown factor with S Class level strength, after reading his impressive track record of rapid notoriety progress. Wondering if Saitama could be a proper challenge to his greater plans later on. After the incident wherein the prisons for upper threat level monsters and mysterious beings had been breached during his and Tatsumaki's clash. Metal Knight came to resent Caped Baldy for his part in the infraction due to damages done to the facility he designed reflecting poorly on the scientists overall capability. But still kept the focus of his ire under tight surveillance like all those standing in opposition of his plans.

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".

Blue
While initially an unknown presence in the hero bracket to the son of Blast, Blue became aware of Saitama as he'd absconded with the, then revealed to be, monstrous Sweet Mask. While such events only furthered his suspicions of the Hero Association He was still impressed by his amazing swift footedness when carrying a mysterious being in tow. After dealing with both a Demon & Dragon Level threat back to back before heading out to deal with another disaster in L-City. Blue arrives only to be flabbergasted by the same hero whom whisked off Sweet Mask defeat the powerful assailant there in one swift punch.

Suiko
Suiko met Saitama when she saw how easily he killed the robot Mirror, and is so shocked by how easily he dispatched it that she pinched her own cheek to make sure she wasn't dreaming. She later on meets with Saitama and wants to exchange some fighting techniques as she acknowledges his power. Later, Suiko introduces Suiryu to Saitama and states that the Caped Baldy is stronger than his brother.

Butterfly DX
Like his contemporary Forte, DX only tagged along with his cockamamie scheme of establishing a pecking order with Saitama only to appease his own showboating. It wasn't until after the former got out of the hospital, said fellow A-Class telling him & Toad about his true capabilities in the interim much to their disbelief, and the battle with Mirror in L-City did he come to realize the veracity of their underclassmen's greater fighting ability.

Forte
Forte initially tried to badger Saitama as his heroic underclassmen but became irate when the latter brought up how getting a rhythm while prepping to fight was too time consuming. After he woke up in the Association headquarters hospital and saw the monolithic brawl between the aforementioned hero and whomever it was he saw him fighting. Forte quickly came to realize just how hilariously outmatched he was and instead opted to ingratiate himself into Bald Cape's good graces by looking after his pets instead.