At the start of the series, Saitama spent most of his time alone being a hero, but after meeting Genos and joining the Hero Association, he gradually gained numerous acquaintances and rivals.
Hero AssociationEditmonster and tried to self-destruct, but was saved by Saitama. After being saved, Genos asked to be Saitama's disciple, to which Saitama nonchalantly accepts, thinking he wasn't 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's lying body after he was defeated by the Sea King. 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. He's also the person who he spends most of his time with, and apparently moved in with him in his apartment. Although Saitama has yet to call him his disciple (as Genos is always the one identifying himself as his disciple), Saitama does train him and gives him relatively good advice from time to time.
Saitama isn't too concerned about Genos' safety, since Genos gets ripped to shreds by monsters at an almost regular basis, with Saitama remaining pretty calm. However, he will save him when he sees him in need; the protectiveness in their relationship is mostly provided by Genos. Whether this is because of Saitama's overall indifference or because of his trust in Genos' abilities isn't clear.Z-City. Bang was amazed by Saitama's strength and tried numerous times to recruit him into his dojo, but Saitama always turns him down. Bang will occasionally invite him to cookouts and vice versa.
King (unintentionally) took credit for killing five mysterious beings that were actually killed by Saitama. This caused King to gain the fame and 'fortune' as an S-classed Hero. Saitama discovered King's true personality and the secret to his success. Saitama wasn't upset or angry at King's secret, and the two would go on to becoming friends. Saitama would go to King's house to play video games, and regularly gets frustrated at losing to King. King is probably one of the few people whom Saitama considers a genuine friend.
Flash mistook Saitama as a monster and the two had a brief scuffle. After clearing the misunderstanding, the two became acquaintances with one another, with Flash acknowledging Saitama's superior abilities.
After being defeated by Garou, Flash was one of the few who regained consciousness and witnessed Saitama defeating Garou. In addition to acknowledging Saitama's strength, Flash decided to teach Saitama some proper fighting techniques.
Fubuki, after her first encounter with Saitama, was sometimes seen along with King and Genos hanging out in Saitama’s abode. She is apparently obsessed with making Saitama join the Blizzard Group after witnessing how powerful he was when she fought him in his apartment and how he fended off Sonic. Her latest attempt was to beat Saitama in a video game competition, which failed due to King’s assistance. Saitama himself declared that he doesn't view Fubuki as a friend, but as an "acquaintance", showing that their relationship hasn't really progressed at all.
After witnessing Saitama goes toe-to-toe with Tatsumaki, her obsession of him only grows immensely stronger as she vows to make him join at any cost.
While the two have had little interaction, Zombieman is one of the only people who is aware of Saitama's true strength after witnessing his fight against Garou, which is related to a warning Genus warned about to Zombieman behind the House of Evolution's reformed life. He asks Genus to remove his limiter as well and make him equal to Saitama, as he feels inferior to the B-Class Hero.
At their first encounter, Tatsumaki didn't think much of Saitama due to his ranking, and would insult him. She was even insulted from him ignoring her. Saitama once mistook Tatsumaki for a little girl due to her small stature, and found her to be annoying.
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Their next encounter would be when they 'fought' in the Psychic Sisters Arc, where Tatsumaki finds out how powerful Saitama really is. Saitama displayed concern and interest toward 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 be even more irritated at her behavior. Tatsumaki came close to revealing her past and Blast to Saitama before deciding not to.
In an audio book it is revealed the two actually met in a haunted house at a festival Saitama worked at. Saitama thought Tatsumaki was a lost little kid, 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 toward each another, and a possible friendship.
Saitama respects and acknowledges Mumen Rider's ability to continuously do 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 the Sea King.
Mumen Rider is one of the few who knows of Saitama's true strength, and treats him respectfully as a fellow hero. He sent Saitama a thank you letter and offered to treat him to dinner, where he thanked Saitama personally, further showing Mumen Rider's humble and respectful nature.
In an extra chapter it is revealed that he and Saitama were once schoolmates, but neither are aware of this fact.
One of Saitama's new neighbors after his move to A-City. Originally Forte thought little of Saitama but after witnessing him go toe-to-toe with Tatsumaki he realizes he can't hold a candle to Saitama.
Originally Sneck thought little of Saitama due to his rank and the fact he almost flunked the exam, but after he heard about Saitama breaking the exams physical records and being swiftly defeated by the rookie, Sneck began to recognize Saitama's abilities. Saitama once unknowingly avenged Sneck when he killed Sea King and defended his honor when he and the other heroes who were defeated by Sea King were being insulted. Sneck was the only person in the Super Fight tournament who was able to figure out he wasn't Charanko (but it did take him awhile).
At first, Sweet Mask treats Saitama with disdain due to his appearance and his ranking. After Saitama kills Sea King, Mask begins to gain an interest in Saitama. Moreover, after witnessing Saitama defeat Garou, he not only fully acknowledges Saitama's tremendous power, but also begins to gain an obsession with him, considering him to have a true beauty of overwhelming power.
Suiryu holds great respect for Saitama not only because of his strength and humble personality but also for the fact they share the same dilemma in finding worthy opponents to fight. Suiryu also admits that Saitama is the only person worthy enough for him to share his disdain of the hero lifestyle in which he keeps hidden in his sub-consciousness.
After being defeated by Saitama from a hip strike, he begins to fear Saitama, not wanting to bump into him again in the future.
After witnessing Saitama defeats both Bakuzan and Gouketsu with little effort, Suiryu not only acknowledges Saitama's god-like power but also becomes very grateful for Saitama for saving his life and goes as far as asking Saitama to make him his disciple like Genos only for him to decline. Furthermore, he also aspired to become a hero from Saitama's heroic action.
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 doesn't mind Sonic's persistence and their continuous 'battles', even going as far to encourage Sonic to do his best. However, he does grow irritated if Sonic starts to bother him during important activities or causing havoc around 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 witnessing the destruction of several of his creations Genus began to study Saitama. He theorized at some point during his training Saitama broke through a limiter that humans have but the price was his hair and emotions.
It is also shown that Genus holds massive deal of respect for Saitama since his effort to successfully remove his limiter to attain godly power, defining all Genus's 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 the way that willpower alone can make humans become absolutely incredible.
Saitama sympathizes with Boros for not finding an opponent worthy of his power. While Boros admits Saitama is far stronger, Saitama has respect for Boros and calls him the strongest opponent he has ever faced.
Choze gained a grudge against Saitama after the latter defeated him in a match, while yawning. In his monster form, Choze believed that he could defeat Saitama. Ultimately Choze would never get his chance for revenge as he was killed by Suiryu soon afterwards.
Saitama first encountered Garou while shopping for a wig and easily defeats him. He then defeats Garou again with a single kick.
The following section contains Webcomic spoilers. You have been warned manga-only readers.
Garou displayed hatred toward Saitama during their fight, seeing Saitama as 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, realizing that Garou wanted to be a hero instead of a villain.
Saitama showed mercy towards Garou, refusing to kill him and acknowledging his flaws.