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.
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.
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.
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.
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.
Nico di Angelo is often underrated. He has such a complex backstory with his relationship to the Underworld and his struggle with his identity. In fanfiction, you could really explore his internal battles more deeply. Another one is Tyson. He's a cyclops and Percy's half - brother, but he doesn't always get as much attention as he could. You could write about his experiences at Camp Half - Blood and how he overcomes the prejudice some of the other campers have towards him.
One underrated character is Tyson. He's Percy's cyclops half - brother. In fanfiction, he could be given more complex storylines, like leading a group of cyclopes in a battle against a new evil force. Another is Silena Beauregard. Her story could be explored more deeply, such as her relationship with the other campers before her sacrifice.
Blitzen is one. He has a lot of potential for further development. His relationship with Magnus could be explored more, like how his dwarven heritage affects their friendship and adventures. Another is Hearthstone. His backstory and his struggle with his family and being a deaf elf could be delved into deeper in fanfiction.
Zuzana could be one. In fanfiction, we could explore her life more in - depth. Maybe she discovers a hidden talent related to the magical world of Karou. She could also have her own adventure that ties in with Karou's story, like getting caught up in a plot by some minor chimaera or seraphim who think she has some information about Karou.
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.