In many percy and his mom fanfiction, Chiron often makes an appearance. As the centaur who trains demigods, he can offer guidance to Percy and his mom. Also, Mr. D, the god Dionysus in his human form at Camp Half - Blood, could be in the stories. His grumpy but powerful presence can add some interesting dynamics, like when he reluctantly helps Percy and his mom because it's in the best interest of the camp.
Grover could be a popular character. Since he's Percy's best friend, he might be involved in the fanfiction, perhaps helping Percy protect his mom or going on adventures with them. Another could be Annabeth. Her intelligence and skills could be useful in the stories, like when they are facing a difficult situation and she comes up with a clever plan.
The bullies themselves would be key characters. They could be the typical mean kids in Percy's school or wherever the story is set.
Jiraiya could be important. He was Naruto's mentor and knew Naruto's father well. He might have some stories about Naruto's parents that could add more depth to the meeting. Maybe he could be there to witness the encounter between Naruto and his mom and share his insights.
Maybe some of the gods or goddesses play important roles. For example, Apollo, the god of medicine, could make an appearance. He might offer some magical healing or at least some advice on how Percy can recover more quickly. This would add an interesting mythological element to the fanfiction.
Percy might become more independent. Without his mom's presence, he has to rely more on himself. He could start to take on more leadership roles within the demigod community. He also may develop a darker side to his personality, as the pain of his mom's death lingers. However, through his experiences and the support of his friends, he gradually learns to balance this new, harsher side with his original kind - hearted nature.
One interesting plot could be Percy and his mom going on a magical adventure together. Maybe they discover a hidden realm full of strange creatures and have to use their wits to get out. Another could be Percy's mom getting some sort of special power and they team up to fight against a new threat in the Percy Jackson universe.
Annabeth is often a popular character. She could be in a story where she has to help Percy deal with his relationship with Poseidon. Maybe she gives him advice on how to handle Poseidon's overbearing nature or how to make the best use of the gifts Poseidon gives him.
In a fanfiction, when Percy reunites with his mom, it could be a very emotional moment. Maybe Percy has just come back from a dangerous quest. His mom would probably hug him tightly, tears in her eyes. She might cook his favorite meal right away, and they would sit down and Percy would start telling her all about his adventures.
He might be initially shocked. I mean, he's used to all the demigod stuff, and then finding out his mom has wizard powers would be a whole new level of surprise.
Minato would be an obvious choice as he was her husband. Jiraiya could also be included as he was Minato's teacher and might have had some interactions with her. Kakashi, being an important figure in Naruto's life, could have some connection to her in the past as well.
I'm not sure specifically since I don't know the exact fanfiction, but it could be his friends who might be either involved in his hacking activities or completely unaware of them. Maybe there's a mentor - like figure who guides him in the world of hacking.