Percy Jackson is a highly empathetic and compassionate character who shows kindness and loyalty to all kinds of people no matter what their background or story is. It didn't matter if they were mortals, immortals, gods, demigods, titans or monsters. He has shown kindness, compassion and loyalty to:
Tyson (his half brother, a cyclops, who he protected from bullies before and after knowing what he was. yes he faltered once but that was more of him feeling bad that he was once again the odd one at camp. he made up for his negative feelings about tyson being his brother by continuously saving him, standing up for him and finally recognizing him his brother. Percy believes so much in Tyson.)
Grover (his best friend, who he protected from bullies before finding out grover is a satyr sent to protect him. he defended grover vs the council of cloven elders, when they labeled grover as a liar regarding his news of Pan)
Rachel (his first mortal friend, who's bravery he admires. His thoughts about Rachel are very positive and he is constantly impressed by her. He has voiced out his respect for her on many occasions.)
Briares (ancient Hundred-Handed One, who was set free from his prison by Percy and his friends. He was abused and so afraid to the point where he wanted to fade like his brothers but ultimately saved chb b/c he was inspired by Tyson's faith in him. and who showed Tyson how to be brave and stick up for those in need? Percy.)
Blackjack (a pegasus who he saved from luke's ship and was so grateful to percy for it that he's always offering percy his help)
Mrs. O Leary (he adopted a literal hellhound for a pet and is fiercely protective of her. he once hesitated to kill hellhounds in TLO b/c they reminded him of mrs. o leary)
Cerberus (literally tells Hades "it wouldn't hurt to play with cerberus once in a while. he likes red rubber balls")
Bessie the Ophiotaurus (sea creature with the potential to destroy the gods but who percy advocated for in front of the 12 Olympians and asked them to save Bessie b/c you shouldn't kill an innocent creature on the basis that they might or might not destroy you)
Other sea creatures (he would lose hours of sleep doing rescue missions)
The river naiad from Geryon's ranch. He literally says he didn't want to be that kind of guy that puts his weight on being the son of Poseidon to get what he wants. He saw that she was scared and was only putting on a brave front to protect her ecosystem. She is one of the reasons why we have Percy embracing what it means to be a son of the sea god - the sea is within him.
He showed her compassion and in turn she tells him a secret. She showed him a way to save his friends.
Calypso (an immortal titanness punished for siding with her father Atlas and the other Titans. He felt extremely bad for Calypso, told her that it wasn't fair that she got punished for siding with her family and asked her how he could help free her. He couldn't stay in Ogygia but he did directly ask Zeus to release her. He also planted the moonshine plant she gave him like she asked "build a garden for me in Manhattan." He remembered her.)
Zoe Nightshade (hunter of Artemis. They had mutual respect after seeing a different side of each other.)
As soon as he figured out it was Hercules who had abandoned Zoe, he threw out his lion-skin cloak, which had been very helpful in keeping him safe from harm. He proceeded to trust Zoe and her judgement and he mourned her death. He actually didn't want to accept that she was dying at first, asking the others to give her more nectar and ambrosia and asking Artemis if she could heal her with magic.
Thalia Grace (the first person to hold her when she was resurrected, who yelled at the other campers to help her and to get her nectar and ambrosia.) For all the times they fought, he also felt protective of her:
Percy put his pride aside to beg Mr. D for help because he looked at Thalia and decided he didn't want to put her in a position where she would die to protect her friends again. Because how could he let that happen to her? He won't.
Clarisse La Rue (felt sympathy for her despite being bullied by her when he first got to camp b/c he recognized what kind of dad Ares was to Clarisse. He gave her the golden fleece, trusting her to complete the quest and save camp:
This even impresses Annabeth. Percy genuinely liked seeing Clarisse happy. He smiled when he noticed her and Chris Rodriguez hold hands at the end of botl.
Artemis (took the sky for her, which they both knew would have killed him, so that Artemis could help Zoe fight Atlas. In turn she vouched for Percy and Thalia when the gods voted on whether they were too dangerous to be kept alive)
Bianca di Angelo (he felt bad for the di angelos. they were taken out of the casino, hunted by monsters and soon after discovering that they were demigods bianca was asked to join the hunt. That is A LOT to process. Percy was the only one who asked her to consider other options before joining the hunters. He also reminded her that her brother couldn't go with her.)
He later was the first to voice out his support as long as she was happy and placed himself in her shoes to better understand where she was coming from:
Percy is the only one who saw Bianca's decision to join the hunters from both her and her brother's perspective. After asking her to consider her options and expressing initial disappointment at her decision, he adjusted his way of thinking and directly asked her how she was doing. How was she settling in with the hunters? How was her life with Nico prior to being discovered? He understood where both siblings were coming from and he tried to comfort and reassure them both (Nico at camp during capture the flag and Bianca during their quest.)
Ethan Nakamura (they were forced to fight to the death in the arena and while Ethan was trying to kill him, Percy only knocked him down and told him to run when he saw an opening. He later asked Ethan to come back with him but he refused. Percy's choice to not kill Ethan and letting him go literally led to Kronos rising b/c Ethan pledged himself to the titan lord right afterwards. Ethan was also the one who figured out Percy's vulnerable spot. He would have killed Percy had Annabeth not taken that knife for him. Percy, even after feeling "betrayed" by Ethan, STILL remembered and considered him when turning down immortality and literally asked for the minor gods to be given recognition - which was what Ethan wanted for Nemesis - and for the gods and demigods who sided with Kronos to be forgiven. Ethan also got his own shroud:
Nico di Angelo. Yes, Nico. Percy spent six months looking for Nico after he disappeared at the end of ttc when percy broke the news to him about Bianca with the goal of wanting to make things right with him. In the labyrinth, he said he felt Nico was close and he ran towards his direction, leaving annabeth grover and tyson running to catch up with him. Nico was double crossed by the owner of the ranch, Geryon ("you should've made me swear on the river styx") and got taken as prisoner to give to Luke Castellan later. It was Percy who bartered for his release:
Percy and his friends were free to go! What does Percy do instead? He makes Geryon a deal to make sure that Nico got out safely.
Also, I find it kinda interesting that Rick Riordan went the "percy talks shit about nico behind his back" route in HOO because this is Percy talking about Nico when he's not there in the og series:
Percy Jackson told the Queen of the Gods that she only cared about her perfect family, not real people, because she didn't secure Nico's safety passage through the ranch. Percy and Annabeth both made a bad impression with Hera because they disliked her attitude towards Nico di Angelo (there's more complexities to this scene but again I won't dive into them).
Percy also literally prayed to Poseidon to help him with Nico:
