Well, aside from Percy and Triton, there could be other characters. For instance, Annabeth might make an appearance in some fanfictions. She is Percy's friend and often gets involved in his adventures. Also, there could be various sea creatures that play important roles, like hippocampi that might help Percy and Triton in their travels across the ocean. And some fanfictions might introduce new original characters, like a young mermaid who becomes their guide in the underwater realm.
In addition to Percy and Triton, there could be characters from the Greek mythology world that are related to the sea. Nereids, the sea nymphs, could be part of the story. They might be friends or foes depending on the plot. There could also be Triton's siblings or other sea gods who have an impact on the relationship between Percy and Triton. For example, if there is a power struggle among the sea gods, Percy and Triton might have to take sides or find a way to stay neutral.
Obviously, Percy Jackson and Triton are the main characters. Percy is a demigod, son of Poseidon, with his own set of powers like controlling water and being a skilled fighter. Triton is a sea god, a son of Poseidon as well, and he has powers related to ruling the sea and its creatures.