Saitama met Genos after the latter was defeated by a monster and tried to self-destruct, but was instead saved by Saitama. After being saved, Genos asked Saitama to take him on as a disciple, to which Saitama nonchalantly accepts, thinking that Genos was not being serious. Though Saitama was initially annoyed by the thought of having an apprentice, he gradually warmed up to Genos, even becoming slightly worried and concerned when he saw Genos' limp body after he was defeated by Deep Sea King, or after Genos got smashed into a rock by Tatsumaki. Saitama holds some respect for Genos since he became an S-Class hero right off the bat, and desires to move up the ranks due to his S-Class status. Genos has become Saitama's companion and friend (or at least close to it). He is also the person who he spends most of his time with, since Genos moved into Saitama's apartment to live with him. Although Saitama has yet to officially call him his disciple (as Genos is always the one identifying himself as his disciple), Saitama actually tries to train him, but since Saitama only knows strength training (which wouldn't help Genos, due to him being a cyborg), most of Saitama's advice/lessons are made up, and only an excuse to keep the cyborg occupied. Despite this, however, Saitama does manage to give Genos some relatively good advice from time to time. Saitama also seems to have enough confidence in Genos to handle certain threats alone, as seen when he accepted Genos' request to fight G4 alone without any pressure (although he asked him if he needs help) and would only help/save Genos, when he sees him in need.
Saitama first met Bang when the giant meteor was falling towards Z-City. Bang was amazed by Saitama's strength and wanted to keep a close eye on Saitama. He then started to spy on Saitama, and was greatly surprised to find out that he was actually Genos' teacher. He then started to think that Saitama himself must have an incredible teacher too, but when he asked Saitama about it, he was again surprised to hear that Saitama doesn't have a teacher and destroyed the meteor with his strength alone instead of using a technique. Bang, realizing Saitama's potential, had since then tried numerous times to recruit him into his dojo, albeit unsuccessfully. Bang also showed great displeasure when Tanktop Tiger and Tanktop Black Hole were trying to drag Saitama into their "newbie crushing", but didn't interfere, since he had faith in him and even declared him to be the strongest person he has ever met, showing a great amount of respect for the (then) C-Class hero. Saitama, while not seeing Bang as a friend and showing disinterest in his martial arts, was still slightly impressed by Bang's abilities from time to time, with the best examples being when Bang defeated him in rock, paper, scissors, and later again when Bang told him how he kicked out Garou after his rampage in Bang's dojo, even admitting that Bang must be "kinda strong". Saitama seems to have some faith in Bang, as he called Bang to assist him in a competition against Fubuki and her Blizzard Group. After receiving his hero name "Caped Baldy", Saitama extremely dislikes it and even compared his hero name to Bang's, expressing jealousy of how cool Bang's hero name "Silver Fang" sounded.
They would occasionally invite each other to cookouts.
They first met when Saitama saved him from Octopus Claw Man during his first one-and-a-half-year of training. While Saitama seemingly forgot about their meeting, King never did, and was wondering for a long time who his mysterious savior was. Saitama would, unknowingly, continue to save King from various monsters, thus prompting King to (unintentionally) take the credit for killing at least five mysterious beings that were actually killed by Saitama. This caused King to gain fame and 'fortune' as an S-Class hero, as well as the reputation as the "strongest man on Earth". At their second meeting, after saving him from Giant Crow, Saitama discovered King's true personality and the secret to his success. King, realizing that it was Saitama who was saving him all this time, expressed guilt for taking credit for Saitama's actions, though Saitama was not upset or angry at King's secret, much to King's surprise and they even started some sort of friendship. Saitama would go to King's house to play video games, and regularly get frustrated at losing to King. Alongside Mumen Rider and (potentially) Genos, King is probably one of the few people whom Saitama considers a genuine friend. When Garou tried to attack King, Saitama would stop the hero hunter with a single kick, saving King again.
King and Saitama also worked together to defeat Elder Centipede, with King luring it towards Saitama so that he could destroy the monster. History would repeat itself and King received credit for killing Elder Centipede.
Flashy Flash and Saitama had a brief scuffle after running into each other at the Monster Association HQ. Flash mistook Saitama for a monster and tried to attack him, but after clearing the misunderstanding, the two became acquainted with one another, with Flash acknowledging Saitama's abilities.
After being defeated by Garou, Flash was one of the few who regained consciousness and was able to witness Saitama defeating Garou. In addition to acknowledging Saitama's strength, Flash decided that he wanted to teach Saitama some proper fighting techniques.
He challenges Saitama to a timed sparring match, in which Saitama would become his disciple if he won. Flash won, but only because Genos distracted Saitama long enough for time to run out. Flash eventually begin to know Saitama well respectfully, once he learned from him and Genos that Sonic keeps hunting the former but keeps being defeated multiple times, realizing Saitama’s power does attract Sonic’s attention as well. His point of view towards Saitama also ends up clash with Sonic’s, who aim to kill Saitama before Flashy’s next.
When they first met, Fubuki was trying to force him into joining her group, as she was worried about her rank. After witnessing how powerful Saitama was when she fought him outside his apartment, how he fended off Speed-o'-Sound Sonic and learning that he had friends in S-Class, she started to admire him and decided to join his group.
She is sometimes seen alongside Genos and King, hanging out in Saitama’s abode. She is apparently obsessed with making Saitama join the Blizzard Group, with her latest attempt being to beat Saitama in a video game competition, which failed due to King’s assistance. Despite knowing that Saitama was superior to her in terms of power, she still doubted that he could actually surpass Sweet Mask and her sister. Fubuki also often becomes irritated by his behavior. For example, when he wasn't listening to her deal and later ditched her with the restaurant bill to chase after Garou, which greatly angered her.
Despite this, however, she still returned the cabbage he left at the restaurant and was ready to forget about their mishap if he apologized to her. Fubuki also shows a softer side of hers towards him now and was also showing a lot of patience with him, something not many people who know him are able to do.
When Tatsumaki asked him what Fubuki is to him, Saitama, after some thinking, declared her to be an "acquaintance", showing that their relationship has not really progressed at all, which deeply hurt her. Despite this, however, he was still ready to protect her from Tatsumaki, showing that he does care for Fubuki to some degree. After witnessing Saitama defeating Garou and later going toe-to-toe with Tatsumaki, her obsession with him only grew stronger, as she vowed to make him join her group at any cost and also admitted that she was wrong about him not being able to reach the top. Thanks to Saitama's assistance in the battle against her sister, and her subordinates' determination to become stronger, she was finally encouraged enough to move up to A-Class.
When Saitama heard of Zombieman's immortality from Genos, he became interested in the S-Class hero and asked him if the rumors were actually true, although Zombieman looked rather displeased by having Saitama around, due to his rank. When Dr. Genus later warned Zombieman about Saitama's strength, Zombieman refused to believe him, even thinking that the former scientist was making fun of him.
After witnessing Saitama's fight against Garou however, he completely changed his mind about Saitama, realizing that everything Genus said about him was true. Zombieman then visited Genus again and asked him to remove his limiter, as well as to make him equal to Saitama, as he started to feel inferior to the (then) A-Class hero. Zombieman was shocked when Genus told him that he can't do so, but the scientist still decided to help him, by showing him something "interesting" in his basement.
In their first encounter, Tatsumaki didn't think much of Saitama due to his ranking, and would regularly insult him. She feels insulted whenever Saitama ignores her. When he first saw her, Saitama mistook Tatsumaki for a little girl due to her small stature and found her to be annoying.
Their next encounter would be when they 'fought' in the Psychic Sisters Arc, which is when Tatsumaki discovered Saitama's true power. Saitama displayed concern and interest towards Tatsumaki, as he ran after her to see if she would be alright flying home with her injuries, and was interested in why Tatsumaki pushes everyone away. However, Tatsumaki simply yelled at him, causing him to get even more irritated at her behavior. Tatsumaki came close to revealing her past with Blast to Saitama, before deciding not to.
Despite her dislike towards the A-Class hero, she still came to acknowledge his strength, even asking him about his name, and when she trapped him underground, she still admitted that this probably wouldn't kill him. After their fight, she admitted that he was "kinda strong", showing that her mind about him was slowly increasing, although she still believed that she would've beaten him in five seconds at her full strength, and also called him an "asshole" for hiding his true strength and for going easy on her during their battle, much to Saitama's annoyance.
In an audiobook, it is revealed that the two actually met in a haunted house at a festival Saitama worked at. Saitama thought Tatsumaki was a lost child, and Tatsumaki called him a balding samurai and thought he was a ghost when her powers didn't work on him.
Saitama and Mumen Rider share a mutual respect for one another, and a possible friendship.
Saitama respects and acknowledges Mumen Rider's ability to continuously meet the weekly quotas of the C-Class heroes, something Saitama found to be a difficult job. He also compliments Mumen Rider for putting up a nice fight against Deep Sea King.
Mumen Rider is one of the few who know of Saitama's true strength and treats him respectfully as a fellow hero. He sent Saitama a thank-you letter for defeating the Deep Sea King, and offered to treat him to dinner, where he personally thanked Saitama, further showing Mumen Rider's humble and respectful nature.
After Mumen Rider got beat up by Garou, Saitama visited him in the hospital and expressed relief to see that he was still alive. Saitama then gave him a banana and asked him for information about Garou.
In an extra chapter, it is revealed that he and Saitama were once schoolmates, but neither is aware of this fact.
Originally, Sneck thought little of Saitama due to his rank, as well as the fact that Saitama almost flunked the hero exam. But after he heard about Saitama breaking the exam's physical records and being swiftly defeated by the rookie, Sneck began to recognize Saitama's abilities. Saitama once unknowingly avenged Sneck when he killed the Deep Sea King and defended his honor when he and the other heroes who were defeated by Deep Sea King were being insulted. Sneck was the only person in the Super Fight tournament who was able to figure out he was not Charanko.
Saitama also saved Sneck from being crushed by Bakuzan.
At first, Sweet Mask looked upon Saitama with his typical disinterest in the lower class heroes. When Sweet Mask first heard mention of him after Saitama killed the Deep Sea King, Sweet Mask's opinion relegates the newbie as having the aura of a B-Class hero, based on appearances via video feed alone.
After witnessing Saitama defeat Garou however, not only does Sweet Mask fully acknowledge Saitama's tremendous strength, but he becomes somewhat obsessed with him, considering him to possess the true beauty of overwhelming power and would even start to compare new heroes to Saitama. Despite his admiration for Saitama however, he showed great displeasure for his refusal to kill Garou and lectured the hero for it.
Beaut was so impressed with Saitama's raw ability, he would opt to take the nameless hero under his wing and make him a national icon far surpassing himself. Believing that Caped Baldy could not only grow to be the kind of superhero the world truly needs but be the kind of hero who inspires others to be the greatest of the great in regards to moral justice and capability as protectors of the Earth.
One of Saitama's new neighbors after he moved to A-City. Originally, Forte thought little of Saitama, seeking to abuse his upper-rank status to pressure him. When Saitama was accidentally insulting his fighting style, he wanted a match with him, only to get hit by a car. After witnessing a seismic event caused by his fighting and later witnessed that the cause of it was Saitama going toe-to-toe with Tatsumaki, Forte realized neither he nor his accomplices could hold a candle to Saitama. He then decided to be accepted by Saitama and to get on his good side, apologizing for his rude behavior earlier and agreeing to take care of Black Sperm and Overgrown Rover for him. Forte also told his accomplices that they wouldn't stand a chance against Saitama, and gave them advice to work on being accepted by him as well.
Speed-o'-Sound Sonic views Saitama as his eternal rival and vows to defeat him. Saitama views their one-sided rivalry relationship in a nonchalant manner. He does not mind Sonic's persistence and their continuous 'battles', even going so far as to encourage Sonic to do his best. However, he does grow irritated if Sonic starts to bother him during important activities, or harms others.
According to Genos, Sonic is one of the few people who recognizes Saitama's true strength. This recognition is what caused Sonic to stop working and continue training in order to surpass Saitama. This recognition and dedication, as well as declaring Saitama a rival and admitting defeat at his hands, indicates (in his own way) a form of respect that Sonic has for Saitama.
After pretending to be Saitama for one day, he gains a new form of respect for Saitama, and vows to never attack him when he's fighting monsters.
Sonic has a very negative outlook on the Hero Association due to the way they look down on someone as strong as Saitama due to his low ranking and appearance. He even criticized Saitama for joining these "so-called worthless heroes" once.
While being disgusted by Boros destroying other planets during his search of a worthy opponent and getting annoyed by his life story, Saitama still sympathizes with Boros for not being able to find a worthy enough opponent that could give him a stimulating fight, thus prompting him to hold back during their fight to give them both an equal and satisfying duel. After being kicked to the moon by the alien and returning to the Earth however, Saitama stops holding back and attacks Boros with more force, which he only survives thanks to his strong regeneration abilities. Boros is also the first opponent that Saitama had to use a serious attack against in order to defeat them. While Boros admits that Saitama is far stronger than him and realizes that he wasn't even nearly using his actual power, Saitama has respect for Boros, and deems him to be the strongest opponent he has ever faced so far.
They first met during the Super Fight tournament. During his fight with Saitama, Choze gabs endlessly about his genetically superior view and eventually foolishly lets his guard down, which causes a bored, yawning Saitama to send him flying to silence him. Choze formed a grudge against Saitama for defeating him and doing so while yawning. In his monster form, Choze believed that he could defeat Saitama.
Bakuzan first met Saitama during the Super Fight, where he soon came to dislike Saitama after getting ignored by him and seeing him struggling to tie his belt. He was puzzled by Saitama's lack of movement and became irate when Saitama did not take on a stance, believing him to be untrained and an insult to fight. When Saitama asks for a demonstration of his strength, Bakuzan complies, detailing exactly how he intends to defeat Saitama, in a brutal fashion. After touching Saitama's head and nearly knocking off his wig, Saitama starts to panic and sends him flying with one punch.
Despite his defeat at Saitama's hands, Bakuzan seemed to have forgotten about him and asked him who he was, when he appeared to save Suiryu and Sneck from him. After recognizing Saitama however, Bakuzan uses all of his moves to kill Saitama, but the hero is unaffected by this. Due to his extreme arrogance, Bakuzan cannot comprehend the fact that Saitama is unfazed from his attacks and deducts that Saitama is paralyzed in fear. In order to finish the fight, he initiates his Hell Dispatch Chop as the final blow, only for Saitama to blow off half of his body, killing him.
The two first met in the hideout of the Monster Association. Orochi was alerted by Saitama's punch against Overgrown Rover and immediately went to look for the hero. Similarly to Boros, Orochi too acknowledged Saitama's power at first glance.
Saitama initially does not want to fight Orochi and says he is just acting in self-defense. After Orochi burns his cape, though, Saitama decides to fight back, warning him he's not going to hold back. After witnessing Rover run away from Saitama, Orochi realizes that the ghost of Z-City was actually him all along. Restless but excited for the upcoming fight, Orochi immediately transforms into his strongest, condensed form. Saitama, on the other hand, is not interested in the slightest about the duel with Orochi. In fact, he even stops the monster during his monologue to tell him that he fought many monsters like him who proclaimed themselves ultimately powerful but still ended up being killed in one hit.
Orochi is so shocked by Saitama's answer he cannot even speak, and decides to begin the fight with a physical attack, using Cross Fang Dragon Slayer Fist on the five dragons of his left arm. After Saitama obliterates the five dragons with a single strike, Orochi stops attacking directly and only uses his powerful laser beams.
Despite Orochi's immensely destructive blasts, Saitama is unfazed. Saitama tries to punch Orochi after a brief dash and is caught by Orochi's dragon-headed horns, but gets free in seconds. After missing the target once again due to Orochi dodging it, Saitama is then hit by a combination of laser beams by all of Orochi's dragons, including his core. Saitama is again unaffected by this attack and defeats Orochi with a simple punch that tears through the monster's entire body. Despite the power of the blow, Orochi is still alive and able to speak, asking Saitama about what hit him. When Saitama tells him it was a normal punch, Orochi's head falls deep underground, recognizing Saitama's strength as terror itself.
Saitama is then told by Flashy Flash that a certain "monster king" lives there and gets excited due to the scary epithet, but he is unaware of his identity and therefore did not realize it was the same monster he already defeated moments before.
After hearing about Garou's power from Mumen Rider and Tanktop Master, Saitama became eager to fight the hero hunter, hoping for a challenge. Saitama first encountered Garou while shopping for a wig. He was sneak attacked by Garou, but the attack was ineffective and Saitama, who mistook him for a robber, easily defeated him with a karate chop. Later on, when Garou attacked King, Saitama was also present and disposed of him with a single kick, still not recognizing him. Garou attacks Saitama a third time after dine-and-dashing from the same restaurant as him, hoping to exact revenge, but Saitama swats him away so as to not look like an accomplice of his.
Saitama is later the last hero standing and able to fight Garou once Garou becomes a true monster. Garou displayed hatred toward Saitama during their fight, viewing Saitama as the personification of the collective unfairness of the world. Adding to that, Saitama is a hero, and represented everything that Garou hated and was fighting against. Saitama is responsible for defeating Garou both physically and mentally, as he forced Garou to realize that he wanted to be a hero instead of a villain, and by crushing his image of a real hero by telling him that he is only a hero for fun. In the end, Saitama showed mercy towards Garou, refusing to kill him and letting him go, being the only one capable of seeing where he fled to.
After witnessing the destruction of several of his creations, Dr. Genus began to study Saitama. He theorized that at some point during his training, Saitama broke through a natural limiter that humans have, but having paid the price with his hair and his emotions.
It is also shown that Genus holds a great deal of respect for Saitama, due to his efforts to successfully remove his limiter to attain godly power, defining all of Genus' ideology about artificial evolution to create better and stronger beings, as well as his desire for eternal life. Furthermore, he respectfully appreciates the fact that Saitama shows him that human willpower alone is able to lift one to incredible heights.
Saitama initially respected Suiryu and his abilities, asking him to show him the fullest extent of his martial arts in order to get a better understanding. However, Saitama became slightly enraged when Suiryu said that heroes are useless and it's pointless to rely on them. This caused him to abandon protecting his identity, revealing his wig, and for the hero to sternly say to Suiryu that if he wanted to live a fun life, he shouldn't become any stronger than he already was. With a deathly glare, he unleashed his Death Punch, destroying Suiryu's clothes.
Despite his initial irritation towards Suiryu, this did not stop Saitama from intervening in the fight between Suiryu and Bakuzan. In fact, when he arrived, he actually complimented the martial artist for holding out for as long as he did. When Suiryu asked Saitama if he could become a hero, the bald hero said that even with his horrible injuries, it was still possible, showing him support for his goal. However, Saitama was not willing to accept Suiryu as a disciple, demonstrating his unwillingness to take on another disciple and help Suiryu with his dream because of the hassle it would bring. Despite this, thanks to Saitama, Suiryu regains his spirit and decides to become a hero like him.