I think Grover is a bit underrated in fanfiction. He's Percy's best friend and his role as a satyr could be expanded upon. For example, fanfiction could focus on his search for the lost god Pan. His abilities like being able to sense nature spirits could be used in more creative ways. Also, Clarisse La Rue. She's often just seen as a bully in the original, but in fanfiction, she could be developed into a more complex character. Maybe explore her relationship with her father, Ares, more and her own journey of self - discovery as a demigod. And then there's Rachel Elizabeth Dare. She has the gift of prophecy and that could be the center of an interesting fanfiction story, showing how she copes with this power and her role in the demigod world.
One underrated character is Bob, the Titan. He has a really interesting backstory and his loyalty to Percy and his friends is quite remarkable. Fanfiction could explore his past in more detail, like his time before he became Bob. Another character is Small Bob, the cat. He may seem like a minor character, but there could be a really fun fanfic written about his adventures in the House of Hades, perhaps with his own little quests. Nico di Angelo is also a bit underrated in this context. There could be fanfiction focusing on his internal struggles while in the House of Hades, like his feelings of isolation and how he copes with the darkness of the place.
One underrated character is Tyson. He has so much potential for a fanfiction focus. He's a Cyclops, which makes him different from the other demigods, and his loyalty to Percy is really heartwarming. A fanfiction could explore his backstory more, like his life before he met Percy at Camp Half - Blood. Another underrated character is Clarisse. She's often seen as just a bully, but there could be a lot more to her. A fanfiction could show her softer side and what made her the way she is, perhaps a story about her first quest and how it changed her.
There's also Grover. He's Percy's best friend, but in a lot of fanfiction, he's just there as a sidekick. He could be developed more, like showing his growth as a satyr and his role in protecting nature. And then there's Clarisse. She's a tough and strong demigod, but often overlooked. Fanfiction could explore her in - depth feelings and her own quests that are separate from Percy's.
Silena Beauregard is also underrated. She could be the focus in a fanfiction set in the school. Since she was a daughter of Aphrodite, she could use her charm and beauty skills in a different context at school. For example, she could use her powers to boost the confidence of shy students or mediate between feuding groups by using her charm to calm everyone down.
One idea could be Percy having to face a new, unknown threat that even the Olympians are afraid of. Maybe it's a primordial force that was locked away long ago and is now breaking free. Another plot could be Percy getting stranded in a different realm, like the Underworld, and having to find his way back while dealing with the local challenges and creatures there. Or perhaps Percy has to team up with some of his old enemies to fight a common, greater foe that endangers both the demigods and the Olympians.
Nico di Angelo is often overlooked compared to Percy. He has such a rich backstory with his connection to the Underworld and his struggle with his identity. Fanfiction could explore more of his inner thoughts and his relationships, especially his friendship with Percy on a deeper level.
Annabeth Chase is also a fan favorite. Her intelligence and strong will make her a great character to explore in fanfiction. Writers might focus on her architectural projects in the context of the demigod world, or her relationship with Percy from different angles.
A powerful demigod who has turned to the dark side and uses his gaming skills to hack into the gods' systems. He wants to rewrite the rules of the world and become a god himself.
Silena Beauregard is an underrated character. In fanfiction, she could be given more backstory and more involvement in the main plot. Maybe she could be brought back in some magical way and have a new role to play in the fight against the forces of evil. Also, the Stoll brothers, Travis and Connor. They are often just used for comic relief, but they could be developed into more serious characters in fanfiction. They are sons of Hermes, so they could have some really interesting stealth and thieving - related adventures that could tie into the main story.
Reyna is another one. She's a strong and independent leader. Fanfiction could focus more on her past, her time leading the Roman camp, and how she copes with the different challenges she faces, especially when it comes to interacting with the Greek demigods.