Hi!
The game itself mentions right at the start that you can either play as Fawna or as an invisible MC but only Fawna will show up in CGs and as a side image since she's the canon protagonist.
If you play as an invisible MC you can choose your name and pronouns, but the invisible MC won't show up in CGs nor as a side image.
If you play as Fawna she will show up in CGs and as a side image, but you won't be able to change her name nor pronouns.
The story itself is pretty much the same. The changes mainly have to do with whether you want your invisible MC to be vegan or vegetarian (or none) and what color you want their hair to be dyed to.
The personality of the invisible MC will be the same as Fawna's, though, since this is not meant to be a self insert character.
Last Edit: Sorry for all the edits, I tried to make it as clear as possible 🙏(and also realized I'd made some typos).
I know some people will be disappointed by this since many like to play as a self-insert or as their own OC. I understand this since I also sometimes like to play as my own character in games as well.
When it comes to writing, though, I don't really enjoy making stories with self-insert characters and prefer when the protagonists are their own established character with their own personality, background and appearance.
If you are a player who prefers to self-insert or play as your own OC and have them appear in the game too, this may not be the game for you 🙏