There could be some Dalek characters as well. Maybe there's a Dalek leader that Harry has to face off against. They would be the main antagonists in the story.
Obviously, Harry would be one of the main characters as it's in the title. He's the one fighting the Daleks.
Depending on how the fanfic is written, there might be other characters from the Harry Potter universe. For instance, Dumbledore could make an appearance in some form, perhaps as a guiding spirit or through a flashback that gives Harry the inspiration or knowledge he needs to fight the Daleks.
Perhaps in this fanfic, Harry teams up with some of his Hogwarts friends. They combine their magical knowledge and courage. For example, Hermione could use her research skills to find a weakness of the Daleks, while Ron provides support and loyalty in the fight. And Harry, being the main hero, directly engages with the Daleks using his powerful spells like Expelliarmus but modified to have an effect on the Daleks' technology.
The Doctor is surely one of the main characters. He's the one who will be opposing the Daleks. And the Daleks themselves are main characters in a sense, as they are the source of the conflict. There might also be some companions of the Doctor who play important roles.
There could be a young and brave goblin warrior who is eager to prove himself in the fight alongside Harry. And perhaps there's a goblin shaman or mage who has special powers and uses them to help in the battles. These goblin characters would each contribute in different ways to the story.
The main character is likely the Emperor of the Daleks. However, there could be other characters as well. Maybe there are some characters from other species who interact with the Daleks. They could be resistance fighters trying to stop the Daleks' evil plans, or some alien diplomats who have to deal with the Emperor of the Daleks. But the core focus would be on the Emperor and those Daleks directly involved with the Emperor's rule and plans.
In this graphic novel, the Doctor is surely one of the main characters. He is the one who usually opposes the Daleks. The Daleks themselves are central, with their evil intentions and their mechanical forms. There could also be other characters like a local inhabitant of the planet where the story takes place. This character could be someone who helps the Doctor or is in need of the Doctor's help against the Daleks. Maybe there's also a character who has some knowledge about the Daleks' weaknesses and can assist the Doctor in his fight. But overall, the Doctor and the Daleks are the most prominent characters in a story like this.
Obviously, Harry Potter himself. He's the main character who is going to fight back.
Obviously, Percy and Annabeth are the main characters. Percy is the one doing the fighting, using his demigod powers which include his ability to control water. Annabeth, on the other hand, is the one Percy is fighting for. She is known for her intelligence and strategic thinking in the original series, and in this fanfic, she is likely the motivation for Percy's actions.
Obviously, Harry Potter himself is the main character. He is the one with the motivation to fight against the Dursleys due to their years of mistreatment.
It could be a group of rogue Death Eaters who have formed their own empire after Voldemort's fall. They might be more radical and have new, even darker plans for the wizarding world.
In 'Harry and Teddy fanfic', the two main characters are Harry and Teddy. Harry is the one who has been through so much in the wizarding world. He has a sense of responsibility towards Teddy. Teddy is a character full of possibilities. He's the product of a tragic event (his parents' death), and Harry's presence in his life is like a guiding light. Their personalities, Harry's wisdom and Teddy's eagerness to learn, are often explored in these fanfics, which makes for very engaging stories.