He will most likely meet Sirius Black. Sirius is an important figure in Harry's life and his presence at Grimmauld Place is quite significant. They could have a deep conversation about Harry's anger and what to do next.
Well, perhaps Harry is angry because of some injustice he faced at Hogwarts. When he arrives at Grimmauld Place, he might find some secrets about the Order of the Phoenix that further fuel his anger. He could also have an argument with some of the residents there, like Sirius or Mrs. Weasley.
Well, in fanfiction, it could be that when Harry arrives at Grimmauld Place, he discovers new secrets about the Black family. There could be hidden rooms or magical artifacts that are crucial to the fight against Voldemort. Maybe he stumbles upon a diary of one of the Black ancestors that reveals a long - forgotten spell or prophecy.
There's a possibility that after Sirius's death, Harry just sort of took over. He had already spent time there and knew the place well. The remaining members of the Order of the Phoenix, seeing his connection to the place and his leadership potential, supported his claim to Grimmauld Place.
Since I haven't read this fanfiction, it's difficult to give a detailed account. However, it may be a story that uses Grimmauld Place as a backdrop for some sort of romantic or sexual plot. It could be about how the characters interact with each other in this location in a more private and personal way. It might also explore the secrecy and the unique atmosphere of Grimmauld Place and how that impacts the relationships and the sexual or romantic elements within the fanfiction.
One key event could be Harry exploring the secrets of Grimmauld Place. He might discover hidden rooms or artifacts. Another could be him hosting Order of the Phoenix meetings there, which would involve a lot of strategic discussions and planning against Voldemort.
One interesting aspect in 12 Grimmauld Place fanfiction could be the exploration of the house's secrets. For example, fanfictions might delve into the hidden rooms and the dark magic artifacts that could be stored there. It's also common to see the characters from the original series interact with the house in new ways, like Harry Potter having a different kind of magical training within its walls.
Maybe Harry has a big fight with Ron over something like Ron's lack of trust. For example, if Ron doubts Harry's intentions again as he did in the original story. Harry, in a fit of anger, kicks Ron out of Grimmauld Place.
It could be that there is no traditional 'antagonist' in the sense. Maybe it's just the situation and the conflict between Harry and Ron's personalities. But if we had to consider one, it could be an outside force that is influencing Ron's actions and making Harry distrust him. For example, a new dark magic object that is affecting Ron's judgment.
In some fanfictions, his arrival is a chaotic scene. There are people running around, trying to secure the place and keep Harry hidden. In others, it's a more solemn moment, with everyone waiting in silence for his arrival, aware of the great burden he carries.