Another character could be a new demigod that Percy meets. This new character could be very different from the ones he already knows, with unique powers and a mysterious past. Percy could write about his first impressions, his growing friendship, and the adventures they have together in his diary.
Grover could also be included. He might accidentally stumble upon Percy's diary and misunderstand some of the entries, leading to some comical situations. And he could also provide some support and advice to Percy based on what he reads.
Annabeth would be a great character to include. She could be reading Percy's diary and leaving little notes or comments in response, which could create some interesting interactions between them.
In Percy son of Neptune fanfiction, including Nico di Angelo would be interesting. His connection to the underworld could bring a new dimension to the story, especially when dealing with any threats or mysteries that involve both the sea and the underworld. Reyna could also be a great addition. Her leadership skills and combat abilities would make her a valuable ally or a formidable opponent, depending on the plot. And of course, Chiron, the centaur, who could offer his wisdom and guidance to Percy from afar, as he has so much knowledge about the demigod world.
Grover could be an interesting addition. Even though he's a satyr, his loyalty to Percy and his own unique skills could be put to good use in a sea - centered story. Maybe Nereids, the sea nymphs. They can bring an element of mystery and beauty to the fanfiction and might be able to help Percy in some way. Another option is Chiron, who could guide Percy from the shore in his sea - related quests.
The gods themselves could be interesting characters to include. For example, Poseidon could offer Percy more power or advice at crucial moments. Athena could also play a role, perhaps initially being an adversary because of her different goals, but later becoming an ally. Even minor gods like Hecate could be included. Her magic could be used to help Percy in situations where his normal powers aren't enough. Also, new characters created specifically for the fanfiction, like a powerful demigod from a previously unheard - of lineage, could add an element of surprise and freshness to the story.
Definitely include Percy and Annabeth. They are the central characters in the original series and their relationship and skills are key. Percy with his water - related powers and Annabeth with her wisdom and strategic thinking.
Annabeth Chase would be a great character to include. She is smart and can help Percy with strategizing in difficult situations. Her knowledge of architecture and ancient history can come in handy when they are exploring new places in the ocean world.
Annabeth would be a great character to include. She could use her intelligence to help Percy solve problems and strategize against their enemies in the underwater world.
Another character that could be included is Clarisse. Her tough - as - nails attitude could contrast with the niece's, and they could start off as rivals but eventually become friends. Also, Tyson, Percy's cyclops half - brother. He could be like a big, protective figure for the niece, and his unique abilities could come in handy during adventures or when the niece is in danger.
Annabeth is a great character to include. She's smart, strategic, and her relationship with Percy adds a lot of depth. Grover is also good. His loyalty and his connection to nature can bring interesting storylines. Nico di Angelo can be a really cool addition too. His powers over the dead and his somewhat brooding personality can create some great drama.
Jason Grace is often included. His leadership skills and ability to fly would be useful in various crossovers. In a crossover with the X - Men, he could be seen as a mutant with his flying and lightning powers. Piper McLean is also a favorite. Her charm - speak ability could be very effective in different crossovers. For instance, in a crossover with the world of Game of Thrones, she could use it to manipulate some of the power - hungry lords. And let's not forget Grover. His satyr nature and his loyalty to Percy make him a great addition to any crossover.
Grover is a great character to include too. His connection to nature and his loyalty to Percy make him a wonderful addition. He can provide comic relief at times, but also be a crucial part of the team when it comes to using his nature - based powers to sense danger or find hidden paths.