Draco Malfoy would be a likely character. Given his Slytherin status, he would likely have a complex relationship with Percy. He might start off as hostile, seeing Percy as an outsider, but then they could end up having to work together for some reason.
Hermione Granger could also interact with Percy. She would be curious about his different abilities and background. She might try to study him like she does with new magical things, and they could become friends as Percy respects her intelligence and she respects his skills.
Grover might also be involved. His ability to sense emotions and his connection to nature could play a role. He could be the one to initially sense Padme's arrival or help her understand the more magical and nature - related aspects of the Percy Jackson world, like the power of the gods and the spirits of the forest.
Percy himself, of course. Given his leadership skills and powerful abilities, he would be the first to step up and face the Pillar Men. His ability to control water could be a great asset or a hindrance depending on the situation.
One character that could betray Percy is Thalia. She has her own issues with the gods and her place in the world. Maybe she gets jealous of Percy's growing popularity and power among the demigods. So, she decides to side with an enemy to take Percy down a notch. Additionally, some minor characters that Percy has helped in the past could be the ones to betray him. They might feel indebted to him but also resentful at the same time. An enemy could manipulate their feelings and turn them into traitors against Percy.
Ares could be a likely character. He's always been somewhat of a wild card in the Percy Jackson series. He might betray Percy for his own amusement or to gain more influence in Olympus. Another could be Zeus. He's often paranoid about his power and might see Percy as a threat and betray him to protect his throne.
Percy Jackson himself is obviously very popular as he is the main character. His friends like Annabeth Chase also tend to be popular as readers are interested in how their relationships develop. Grover Underwood is another one that fans like to see in these fanfictions, especially when he uses his satyr abilities to help Percy on his adventures as the heir of chaos.
Issei would be really interesting. He's the main character in DxD and has his own set of powers and a rather unique personality. His pervy nature would contrast with Percy's more heroic and sometimes serious demeanor. Their interactions could be both comical and lead to some great character development for both.
His mother, Sally Jackson, could play an important role as an ally. She has a strong will and has always been there for Percy. Also, Tyson, his cyclops half - brother, could be a powerful ally with his strength and loyalty. And don't forget about the Nymphs. Some of the water nymphs might help Percy since he has a connection to water.
Probably other demigods would interact with Percy. Maybe some demigods who are in need of a more stable influence in their lives would seek out Percy. Also, characters like Grover might find Percy in Hestia's care and there would be a reunion. And the gods and goddesses who are associated with the concept of home or family in Greek mythology, like Demeter, could also have interactions with Percy.
It could be Percy and Black Widow. Natasha is a master of reading people, and she'd notice Percy right away. She might approach him in a more stealthy way compared to the others, trying to assess if he's a threat or an ally. And Percy, being used to danger, would be on his guard at first, but then start to trust her as she shows her good intentions.
One interesting plotline could be Percy having to deal with the Slytherin values while still holding onto his own heroic nature from the Percy Jackson series. Maybe he has to face prejudice from some Hogwarts students who are suspicious of a 'half - blood' being the heir.