Obviously, Harry Potter himself. He's the main character who is going to fight back.
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.
Ron would also be there for Harry. His loyalty and friendship mean he'd be by Harry's side, maybe even using his own family's magic - related knowledge to assist. And of course, Dumbledore. He has a vast amount of magical knowledge and wisdom, and he'd probably offer some crucial advice or perform a spell to help Harry.
Marty McFly and Doc Brown are most likely to be the main characters. They are the central figures in the 'Back to the Future' franchise, so it's expected that they would be the focus in this fanfic set in 2015.
Obviously, Harry would be one of the main characters as it's in the title. He's the one fighting the Daleks.
Obviously, Harry Potter himself is the main character as he is the one getting back at the Dursleys. The Dursleys - Uncle Vernon, Aunt Petunia, and Dudley - are also main characters as they are the targets of Harry's revenge.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
Perhaps in this 'harry fights back fanfic', Harry discovers some new powers or spells that he uses to fight back. He could be tired of being pushed around by Voldemort's followers or other bullies in the magical community, and he uses these newfound abilities to protect himself and his friends. For example, he might learn an ancient spell from a hidden book in the restricted section of the library. This spell gives him an edge in combat and he starts to turn the tables on his adversaries.
Obviously, Natsu and Lucy would be the main characters. They are the core of the 'nalu' relationship. Their relationship is central to the fanfic, and the family aspect would likely revolve around them.
Hermione would be a very likely ally. She's smart and resourceful, and could help Harry figure out the best ways to use magic against the Chitauri. Ron might also be there, with his loyalty and fighting skills. Additionally, some of the more experienced wizards or witches from Hogwarts could potentially join in to help Harry.
Obviously, Harry Potter himself would be a main character. Elrond, who is raising Harry, would also be a central figure. There might also be other elves like Arwen, Elrond's daughter, who could play important roles. Maybe some of Harry's friends from Hogwarts could make appearances in the story, even if it's just in his memories.