The main character is Harry, of course. But in addition to Ron and Hermione, other characters could be significant. For example, Voldemort or his followers might sense Harry's new power and become interested in using it for their own evil purposes. So they could be antagonists in the fanfic. Also, some new characters might be introduced, perhaps a wizard or witch who has some special knowledge about the Obscurus and tries to guide Harry on his journey, either for good or bad motives.
In some versions of 'harry becomes an obscurus fanfic', the key plot could be Harry's journey to master the Obscurus power. He might start out as an outcast because of it, but then discovers that it can be a weapon against the real threats in the wizarding world. His relationship with Dumbledore or other authority figures might change as they try to help or restrain him. And there could be a big showdown where he has to decide whether to fully embrace or reject the Obscurus within.
Obviously, Harry would be one of the main characters. Since it's about him becoming death, he's at the center of it. There might also be his friends like Hermione and Ron. They could be reacting to Harry's transformation, either trying to help him or being afraid of the change in him.
Obviously, Harry Potter himself is a main character. Since he is becoming Lord Potter, he is at the center of the story. Hermione and Ron are likely to be important characters as well, as they are Harry's closest friends and will surely be involved in his new journey as Lord Potter.
The members of Wildwind would surely be important characters. They could interact with Harry, maybe give him backstage passes or even ask him for his opinion on their new songs.
Ron could also be a major character. He might have a hard time accepting Harry as a girl at first. But as the story progresses, he could become more understanding and protective of Harry in this new form. And there could be some really funny and awkward moments between them.
One interesting 'obscurus harry potter fanfic' could be about Harry learning to control an obscurus within him in a way that the original books didn't explore. Maybe he discovers ancient magic that helps him tame it and use it for good, like protecting Hogwarts from a new dark threat.
Obviously, Bella is a main character as the title implies she is the one becoming a vampire. The Cullen family members are also likely to be main characters. Edward, who was in a relationship with Bella when she was human, will surely play an important role. Also, Carlisle, the patriarch of the Cullen family, might be involved in guiding Bella through her new vampire existence.
I'm not sure specifically as there could be many different takes in such a fanfiction. It might explore Harry's journey if he was an Obscurus, perhaps dealing with the dark and uncontrollable magic within him and how it affects his relationships and his role in the wizarding world.
Obviously, Harry Potter is the main character. His journey as he becomes Lord Potter and how it changes his life is the central focus.
Obviously, Albus would be one of the main characters as the title implies he is the one who 'becomes' like Harry. And Harry himself would also be a central figure, either as a sort of inspiration or even in flashbacks.