Malwarebytes keeps saying AOEO installer is a virus

As long as you follow the links from either the reddit guide or the install guide on this site, the downloads should be clean.

Both of these guides are the same.
Reddit: https://www.reddit.com/r/projectceleste/wiki/how-to-install-and-play-aoeo-celeste-fan-project
Site: https://www.projectceleste.com/install/

If you have any other questions or need further help, hit the chat button in the menu bar at the top and join the #help channel in our discord.