Can you put this game on Steam please, the entire game requires somewhere around 900GB which is what I do not have.
900GB? That doesn't sound remotely right, the entire unzipped folder is no more than 3GB. I have no idea how it would possibly ask for 900, I don't know if there's much I can do to help you there. As for the Steam question, I currently prefer leaving it on Itch as I'm unfamiliar with the process for Steam. If I continue working on the game and it eventually gets to a more robust state I can be happy with, I might move it to Steam. For now I prefer the casualness and freedom of Itch.
Good to hear! I plan to add more characters, behind the scenes the game uses VRM which is a common format. Though it's not quite robust enough to allow users to import their own VRM files. I mostly plan to add characters of friends that already have models. So sorry to say, but I likely won't be modelling your character. At some point in future development I might look into allowing runtime VRM import.
Pretty sure Steam let's about anything that isn't straight up illegal or a virus onto Steam. If I remember correctly it DOES cost $100 USD no matter if it's free or not though.
No clue how to actually go through the process of getting the game ON to Steam, but the $100 USD cost means you should probably wait a bit anyways.