Heartwarming: One Punch Man

  • During the "House of Evolution" arc, Saitama remains his eternally bored self up until a genetically altered rhino turns his new pretty-boy cyborg friend into wall art. He immediately gets pissed.
  • In a bonus chapter that takes place before Saitama lost his Shonen Hair, he follows a group of heroes thinking that they would lead him to the Monster of the Week that's been attacking lately. When he find out that they lead him to a citizen defense shelter instead, he makes to leave. The surrounding citizens think he's having a panic attack and trying to commit suicide. So they all ban together to hold him back, just so they can save him from what they think is certain death. All the while stating that he's young and that he has so much to live for.
  • In a bonus chapter, a nameless B-Rank hero is disheartened by the hero system and believes that normal people like him will never make it big. In comes Saitama who ends up beating a monster that not even Tornado could defeat completely using only his normal strength. He tells the hero that the only person keeping him back is himself and that he can truly be strong if he only works hard for it. Saitama ends up inspiring the nameless hero to leave Blizzard's gang and strike out on his own.
  • Licenseless Rider standing and fighting against an opponent that can kill him in an eyeblink is a crowning moment of awesome. The crowd of civilians watching cheering him on, even though they know that there is no way he can win? Heartwarming. Even Saitama is impressed by his Heroic Resolve.
    "You did well. Nice fight."
  • Genos' description of Sweet Mask is that Genos would have been like that, had Saitama not been in his life. His Sensei may not have taught him special abilities, but he did teach him the most important power.
  • Saitama making himself out to be a jerk in front of the civilians that watched him defeat Sea King... so that the heroes who had risked their lives and nearly died would get the recognition for having "weakened" the opponent beforehand. Bear in mind, Saitama became a member of the Hero Association for the primary purpose of getting the recognition he deserved for repeatedly going out and defeating the Monster of the Week... so him giving up that chance for the sake of others getting the recognition they deserve is truly heroic.
  • "B-Class". Saitama's starting to get hate mail, and the Hero Association are questioning whether he's for real, but there's someone aside from Genos who wants to thank him for what he's done. It's from Licenseless Rider, who later treats him to oden.
  • In one of the bonus chapters, Saitama gets picked up by the police for generally being suspicious. During his interrogation, a demon-level monster strolls into the station and lays waste to the officers there. The chief swallows his pride and asks the Hero Association, which the police regularly butt heads with, for help. The man in charge there rubs it in and makes the chief beg for it. Saitama smashes the phone before he can.
    Saitama: Heroes and Police don't do their jobs because someone says 'please.'
    • A large number of Heroes bribed by the Hero Association show up at the station to kill the monster and humiliate the police. Then a bald police officer comes out the door dragging a dead monster...
  • Another instance is when a kid offers Saitama his allowance money for saving him. Saitama accepts, and returns it telling the kid it's alright.
  • The omake after chapter 36 adds a moment where just before Metal Bat and Handsomely Masked Sweet Mask can fight, Metal Bat gets a phone call from his little sister, telling him how her piano recital went. He goes from cocky Jerkass to Jerk with a Heart of Gold in an instant, even asking Sweet Mask for an autograph to make it up to her for having missed it.
    • Heck he doesn't even seem to hate Sweet Mask anymore either!
  • Another instance is Fubuki in the later parts of the webcomic. She cared for her old friend Psykos to the point that she would fight Tatsumaki in order to protect her. And not just Psykos but also her subordinates. One could say that spending time with Saitama gave her the courage to do that.
  • Speaking of Tatsumaki, her Big Sister Instinct for Fubuki should count for a lot. Her Berserk Button is absolutely mashed when Garou says he's killed her (not that it helped beat him at all) and in flashback it's shown that she used her powers against the kids that bullied Fubuki for hers.
    • The heartwarming has started to fade a little now that it's becoming clear that Tatsumaki is actually OVERprotective, and is planning to force Blizzard to stop being a hero and basically keep her locked up to keep her safe after Blizzard's showing during the Prison battle disappointed her. And that Tatsumaki has basically been ruining her life through her overprotective tendencies since they were kids.
      • And now getting into Tornado's past has made it even worse Tornado having been isolated from her family when she was a child and forced into experimentation. While she does genuinely love Blizzard, her actions are completely out of hand and clearly a result of her being bitter at the world.
    • For what it's worth, it did go back to being moderately heartwarming seeing Tatsumaki acknowledge Fubuki's strength, express joy at the sight of her progress and change her mind about forcing her to stop being a hero. If only she hadn't immediately decided to force the rest of the Fubuki Group to quit being heroes instead.
  • Garou's relationship with a small, homely boy who's a fan of heroes.
    • After being rescued from the Monsters Association, the kid pays Garou back by standing up to Sweet Mask when he tries to execute the supposed "Hero Killer".
  • Saitama visiting Licenseless Rider in the hospital and offering him, Tanktop Master and Charanko bananas.