His former teacher Bang taught Garou everything he knows about fighting. After Garou went on a rampage and attacked his fellow students Bang kicked him out of his dojo. After he heard Garou started hero hunting Bang recruited his brother and went out to stop Garou.
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Garou does not respect Bang as a person, but he admires Bang's martial arts, even copying a combined technique of Bang and Bomb as his own. Despite the villainous acts he has committed, Bang ultimately still cares for Garou. Garou recognizes Bang's lingering affection for his former student from how different his punches were when he was expelled. It is unknown what Garou now thinks of his former mentor after his defeat.
Garou displays hatred toward Saitama during their fight, seeing Saitama as the collective unfairness of the world. Adding to that, Saitama was a hero, representing 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.
Garou encountered Metal Bat on his way to fight Watchdog Man. At first Garou doesn't think highly of Metal Bat, but later admits that Metal Bat could defeat him with one clean hit if he could've landed it.
A young boy Garou met during his hero hunt. Garou saved Busaiku from Royal Ripper. It is safe to assume Busaiku is Garou's only true friend.
Zenko interfered between Metal Bat and Garou's fight. She demanded Garou and her brother to not fight in front of her because Metal Bat promised her not to. Garou was initially against her outburst but finally decided to accept her proclamation.
Similar to Busaiku, Garou seems to have a level of care for Zenko because she's a child.
He stopped Sludge Jellyfish from attempting to kidnap Zenko. While his motivations in engaging the monster was because he was being observed and wanted to confront his observers, he only engaged Jellyfish after he was about to kidnap Zenko. Further, despite how he was in a rush to find Watchdog Man, he stayed behind to watch the monsters leave the area and away from the unconscious Metal Bat and Zenko.
While they have never met, Suiryu has wanted to fight Garou since his victory in Super Fight.