Thank you so much for playing!!! If you're interested, you can check out the postmortem for the game for a little more detail about my thought process while making it! As for your question:
===SPOILERS===
I mostly left Geist's identity up to player interpretation, since I didn't think that whether he is "real" or not significantly changes what I want people to get out of the story 😂 and I wanted to give people the freedom to interpret it how they wanted without being bogged down by some kind of "canon" answer!
In my own headcanon, Geist is just a manifestation of the protagonist's mental problems - mostly because I think it would be a total bummer to have a gaslighting ghost hanging out for your entire life, third-wheeling when you just wanna hang out with your bf 😂 However, I also did give Geist a real "ghost backstory" that I detail in the postmortem - mostly because well, he has a romantic ending, and I didn't want to tell people "well that ending isn't real because Geist isn't real!!!" You can end up in a codependent relationship with the ghost if you want to!!! 😩🙏
tl;dr Feel free to interpret "Geist" how you want, there isn't a wrong answer! If you want my "author headcanon," Geist is indeed a manifestation of the protagonist's anxieties and doesn't exist in real life.