It's possible. There could be other demigods or mythical creatures. Maybe a centaur who gives Percy some advice regarding the naiads. Or a sea monster that they have to face together.
There might be some minor characters like a local fisherman who has seen strange things around the area where the naiads live. This fisherman could play a role in providing some background information about the naiads to Percy. Also, there could be a rival demigod who is jealous of Percy's connection with the naiads and tries to disrupt their relationship.
In some fanfictions, Grover, Percy's satyr friend, might also be an important character. He could accompany Percy when he encounters the Naiads. And there could be a particular Naiad that stands out more than the others, perhaps one with a unique personality or special powers. For example, a Naiad who has the ability to control not just water but also the plants around her water source.
I'm not sure specifically as I haven't read this particular fanfiction. But it might involve Percy having some sort of adventure or encounter with the naiads. Maybe they go on a quest together or there's some mystery in the underwater world that they have to solve.
One idea could be Percy helping a Naiad protect her water source from pollution. He uses his demigod powers to fight off the forces causing the pollution.
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.
Piper McLean could be a great addition. Since she has some connection to Aphrodite already. She could either help Aphrodite in her plans or try to protect Percy from her. And of course, we could add some minor gods or goddesses like Eros. He could be Aphrodite's little helper in creating chaos with love arrows, which would definitely fit in with the lemon aspect of the fanfiction if it involves some love - related mischief.
One character could be a time - keeper or a wizard who has the power to send Percy back in time. This character could be an old and wise figure, full of knowledge about the past and the ways of time travel. Another could be a historical figure that Percy meets in the past. For example, a famous warrior or a philosopher who becomes Percy's ally or mentor.
One interesting character could be a punk - inspired Annabeth. She might have a new, edgier side to her personality, like being more into street - smart strategies. Another could be a punk version of Grover, with a really cool and rebellious attitude towards the nature spirits. And there might be some original characters created in the fanfiction, like a punk demigod who has a unique power related to music or graffiti.
Besides Percy, his agent could be a character. The agent would be responsible for handling Percy's endorsements, scheduling his competitions, and dealing with the business side of his athletic career. There could also be a physiotherapist, especially if Percy gets injured during his training or competition. This person would help him recover and get back to peak performance. Another possible character is a fan. A die - hard fan of Percy who follows his every move, collects his memorabilia, and is always there to cheer him on, either at the stadium or through social media. And there could be a local sponsor who is investing in Percy and hoping for his success at the Olympics.
There could be a rival within the family. Someone who wants Percy out of the way, perhaps for inheritance reasons or because they're jealous of Percy's abilities or popularity. This rival could be the one who instigates the events that lead to Percy's disownment. And then there might be a mentor - like figure outside of the family who takes Percy in after he's disowned and helps him to grow and face his new situation.
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.