Typically, Harry Potter would still be one of the main characters, but his role would likely be significantly altered due to the presence of the 'god emperor'. The 'god emperor' himself would be a central figure, of course. Hermione Granger might also be a main character, as she often plays a crucial role in the original series and would likely be involved in the new power structure. Ron Weasley could also be there, perhaps as a supporter or an opponent of the new regime.
In this 'harry potter god emperor fanfic', the 'god emperor' is the most obvious main character. Harry Potter would still have a major part to play, but he might be more of a foil to the 'god emperor' this time. Hermione Granger would be important as she would try to analyze and understand the implications of the 'god emperor' rule. Ron Weasley would be there, and depending on how the story goes, he could be a voice of the common wizard in the face of this new powerful authority. There could also be other characters from the original series like Dumbledore (maybe in flashbacks or as an influence on the current situation) who are relevant to the main plot.