Obviously, the new girl would be a main character. She could be someone with an unknown parentage at first, like many demigods. Another main character could be the camp director, who would be guiding her and trying to keep her safe. There might also be a friendly camper, like a son or daughter of Hermes, who takes the new girl under their wing and shows her around the camp.
The new girl is the main focus. However, her cabin mates could also be important characters. They could be initially hesitant to accept her, but as they get to know her better, they become her friends. There might be a mysterious figure in the woods near the camp who seems to be watching the new girl. This figure could turn out to be either a friend or a foe later in the story. And don't forget the camp's centaur trainer, who could play a crucial role in teaching the new girl combat skills.
The new girl herself is the central figure. But then there could be a rival character, maybe a more experienced demigod who is jealous of the attention the new girl gets. Also, there could be a mentor - like character, perhaps an older demigod who has been through a lot and can offer wisdom to the new girl. And of course, there could be a love interest, a fellow demigod who she has a romantic connection with as the story progresses.