U.A. High School Students
Stain despises Tenya for his desire for revenge against him. He scolded the young hero for prioritizing his selfish desires to revenge his older brother over saving the Pro Hero he was about to kill. Even after Tenya learned from his mistakes Stain stopped at nothing to kill him, claiming that people do not change their views easily.
Tenya, though he hates him immensely for paralyzing his brother, acknowledges Stain was right about him and resolved to start over as a Hero.
Stain, like All Might, respects Izuku for his convictions and bravery he displayed in his fight with him. He acknowledges Izuku as a true hero for protecting others in spite of the danger. Because of his respect, he both spared Izuku when he first fought him and risked his life to save the young hero-in-training from a Nomu.
Izuku seems to also share a similar respect for Stain. When speaking with Shigaraki at the mall, Izuku confessed that while he didn't accept Stain's methods, he did at the very least understand Stain's desire and the ideals Stain stood for. Izuku also seems to be grateful towards Stain for saving his life from the Nomu.
Because of their encounter, Shuichi Iguchi prevented his comrade Magne from killing Izuku declaring that he was the person that Stain saw fit to save thus being a man worthy of the title of a true hero.
As with Izuku, Stain seems to also respect Shoto for his skills and convictions as a true hero, although this does not prevent him from attacking him with the intention of seriously injuring him, and criticizing him for relying too much on his Quirk.
Shoto, on the other hand, despises Stain for his ideologies about "fake" Pro Heroes, considering him nothing more than an anachronistic and murderous fundamentalist, and defending Iida from his insulting criticism and his attacks.
At first, Shigaraki wanted to recruit Stain for the League of Villains, but after explaining his motives and objectives, Stain views Shigaraki as an immature child who wants to cause needless bloodshed all for nothing. He did not hesitate to pin him down when they conversed and refused to join his alliance, which earned him Shigaraki's hatred. However, he was amused by Shigaraki's desire to kill All Might and destroy those who worship that caused him to see the latter had potential to grow.
Shigaraki also despised how Stain's capture was the main news story instead of the Nomu attack. After having a conversation with Izuku, Tomura discovers his true hate for Stain comes from his connection with All Might. Wishing to create a world without All Might, Tomura develops his own conviction and states he will use Stain's ideology as a stepping stone for himself.
When he was Stendhal, he formed a partnership with Kuin, in which both benefited from each other, although that was not to mean they were really allies, as Chizome warned Kuin she will be judged by "Stendhal" sooner rather than later.
She helped Stendhal with the extermination of the Abegawa Tenchu Kai, and in return she demanded him return the favor by taking out specific targets for her. He obliged to Kuin's demand since even the lowest of filth has its worth.
After being defeated by Knuckleduster and being enlightened by his words, Chizome understood that what he lacked was resolve. Kuin Hachisuka makes fun of him about his need for resolve, for which he throws a knife that sticks into her eye hidden by the patch. Giving her for dead, he walked away without realizing that Kuin blocked his attack. Despite this, Kuin decided not to retaliate, considering since he will makes things all the more interesting.
Chizome seems to have a great deal of respect for All Might and even states that he is the only true hero in the world.
His respect towards him came from his respect he had towards him when he was a teen, All Might was the reason Chizome initially decided to become a Hero. Chizome's respect and admiration for All Might runs so deep that before finally falling unconscious, Chizome declared that only All Might is allowed to defeat him.
Although it does not seem that they have met previously, Stain has an huge hatred and holds contempt towards Endeavor, considering him the epitome of everything that a hero should not be. When they meet, Stain enrages and challenges Endeavor and other "Fake Heroes" to try and defeat him in battle, releasing a malicious aura that paralyzes everyone with fear, even making Endeavor step back.
While he was still Stendhal, at first he respected the young Koichi for his dedication to being a Vigilante. He once saved Koichi’s life from a Instant Villains. When the villain runs away, Koichi thanks him for saving him and asks him his name. Before leaving, Chizome reveals to Koichi that he is called Stendhal and is a fellow vigilante. This meeting made a great impression on Koichi, who is star gushing over Stendhal.
But this changed when Chizome tried to kill Soga. At first, Stendhal warns Koichi not to get in his way as there is no value in saving Soga since all he deserves is a dog's death. But Koichi tries to save Soga and run away with him on his back. Stendhal praises Koichi's innate act of goodness but still considers him evil since those who side with evil have evil within themselves, and tres to kill him too until Knuckleduster arrives to save them.
Apparently, Chizome respect the "Vigilante of Naruhata", not wanting to fight against him when he arrived just in time to save Koichi Haimawari and Soga Kugizaki. Stendhal saw him as someone who is forging the path of righteousness in this crooked society and as a result they shared similar ambitions. However, Knuckleduster disagreed totally with his opinion, viewing himself as completely different to Stendhal: he is a champion of justice while Stendhal is just an insane street murderer.
Disappointed by the fact they cannot reach a mutual understanding, Stendhal attacked Knuckleduster, but got knocked by out him. Stendhal's nose was completely smashed in and he was forced to retreat, causing minor injuries on the Vigilante. However, his defeat was not a complete loss, as he became enlightened after hearing Knuckleduster's comments about resolve. Stendhal thanks him for this revelation before he retreats.