You can be a Hashira, a Demon, a fellow demon slayer, anything! You create the story and shape it the way you want! If you decide on Option B though you're forced to go to the timeline after Hantengu (Upper 4) and Gyokko (Upper 5) we're killed. Also, suggestion for those reading this. I highly suggest reading the manga books to unstand the concept and all, borrow them at the library or smth!