Dumbledore would also be a main character in most cases. As the leader of the secret order, he would be guiding Harry and the others, making important decisions, and using his vast knowledge and wisdom to deal with the threats against the wizarding world.
Obviously, Harry Potter himself is a main character. He is likely at the center of the story as he usually is in any Harry Potter - related plot. He'll be the one facing the challenges and growing as a wizard.
Perhaps a plotline where the secret order discovers a hidden prophecy related to Harry that changes their whole strategy for protecting him. They then have to race against time to figure out how to use this new information to their advantage.
It's hard to say for sure without reading it, but it could be Harry Potter himself. He often finds himself in the middle of things and might be involved in establishing or dealing with the power of order. Hermione could also be a main character as she is intelligent and would likely play a role in any new system of order. Ron might be there too, providing his loyalty and courage.
It's likely that Harry Potter himself would still be one of the main characters. Hermione and Ron would probably also play important roles. However, there could be new characters introduced as well that are central to the new order concept.
A character from the Percy Jackson series could also be a potential addition. Maybe Annabeth Chase. She is known for her strategic thinking and knowledge of ancient magic. In a secret crossover fanfic, she could help Harry and the Order of the Phoenix plan their battles more effectively. She could bring in knowledge of different magical artifacts and how to use them. Her presence would be a secret weapon for the Order, as she could offer insights that are not part of the normal Hogwarts curriculum and help in the fight against Voldemort.
Since it's a fanfic, the main characters could be original witches created by the author. Or it might focus on some of the known witches from the Harry Potter series like Hermione (although she's a witch - wizard hybrid in a way), and expand on their stories in relation to the secret.
Albus Dumbledore can be a key figure in these fanfictions. He could be the one assigning Harry his secret agent tasks or guiding him from behind the scenes. Severus Snape may also play an important role, either as an ally or a suspect, given his complex history in the original series.
There might also be new characters created specifically for this fanfiction. Maybe a young, talented witch or wizard who Harry discovers and brings into the Order. This new character could bring fresh ideas and a different perspective to the group, while also having their own story arc within the fanfiction.
Obviously, Harry would be one of the main characters as he is the one doing the cooking. Members of the Order like Sirius Black could be main characters too. He might be the one who encourages Harry to cook for the group or gives him some special cooking utensils or ingredients from his family's collection.
Among the popular characters in 'Harry Potter Order of the Phoenix' fanfiction are the Weasley twins. Their pranks and their role in the fight against Umbridge and the Death Eaters can be elaborated on in interesting ways. Also, Ginny Weasley is popular. Her growing relationship with Harry and her own strength as a witch can be explored more deeply in fan - made stories.
Hermione Granger could be one of the main witches. She's extremely intelligent and powerful, and in the fanfiction, she might be involved in some secret magical research or adventures.