Valerie is another popular character. She has her own vendetta against ghosts and her interactions with Danny, especially when she doesn't know he's half - ghost, can create a lot of tension and interesting plotlines. In high school fanfiction, she might be competing with Danny in academics or sports while also hunting ghosts on the side.
Obviously Danny himself is very popular. He's the main character and the source of most of the action and drama. His two best friends, Tucker and Sam, are also often featured prominently. They provide support and comic relief in many fanfictions.