Perhaps a half - human, half - nymph character. This new character could have a special connection to nature, being able to communicate with plants and animals in ways that even Percy can't. In the context of a champion - themed fanfiction, they could be a wild card. They might not be fully trusted by the demigods at first because of their nymph heritage, but prove to be an invaluable ally in the end, using their unique abilities to help the champions in their quests.
A new character could be a demigod with the power to control dreams. This character could be both an ally and a potential threat. They could use their powers to help Percy by giving him important information through dreams, but also could be manipulated by an enemy to create nightmares for Percy and his friends, causing them to be sleep - deprived and less effective in battles.
Percy himself is always interesting. In fanfics, he might be even more complex as a champion. Another interesting character could be Annabeth. In some champion - related fanfics, she could be the strategist behind Percy's success as a champion. And then there's Nico. His dark powers and his own journey could intersect with Percy's in interesting ways in these fanfics when Percy is in his champion role.
It might be a mortal who has been given special powers by the gods for a specific purpose. This mortal could be an unlikely hero, but through a series of events, they are chosen to be the 'Champion of the Sun'. They would then have to learn to control their new - found sun - related abilities and face the challenges that come with the title.
Another antagonist could be a powerful frost giant. If Percy is involved in protecting Asgardian interests related to Thor, a frost giant could be a major foe. They are strong and have their own motives for opposing Thor and his champion. They might see Percy as an easy target at first but soon realize his strength.
Well, there could be some ice - related monsters or creatures that serve Khione. For example, ice giants or frost - breathing serpents. These could be constantly attacking Percy or trying to prevent him from achieving his goals while he's Khione's champion. Also, there might be some jealous demigods who think Percy is getting special treatment from Khione and they become antagonists by trying to sabotage him.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.
Maybe a new character who is a half - nymph. She has a special connection with nature and can make plants grow in the middle of the gymnasium during class. This would make for some really interesting scenarios in the gym class. For example, she could create a maze of vines for a race. Or there could be a character who is a demigod from a very minor god that we haven't heard much about. He has a unique power that he discovers during gym class, like being able to turn invisible for short periods of time which gives him an edge in certain activities.
Well, Percy himself is always a memorable character. His courage and quick - thinking in the face of magical challenges make him stand out. Also, any new characters that are created in the fanfiction, like a powerful wizard who mentors Percy in the ways of magic, could be very memorable.
I think a character with the ability to control time would be really interesting. Imagine Percy having to deal with a demigod who can accidentally mess up the time continuum. It would create all sorts of wacky situations at Camp Half - Blood. Like, one moment they're having breakfast, and the next, it's suddenly midnight because the adopted demigod made a time - related mistake.
One of the most intriguing characters is Annabeth. In the context of Percy being the champion of Mars, Annabeth could be in a tough situation. She might be worried about Percy changing because of his new role and she has to figure out how to support him while also dealing with her own feelings. Another interesting character could be Grover. Grover's role as a satyr might bring in some unique elements. He could be the one who discovers some hidden lore about Percy's connection to Mars that no one else knew.