Hi! I assure you you're not bothering me at all, in fact, I'm glad you asked these questions before playing the game. Unfortunately I'm not too familiar with the topic of plural systems, but hopefully this reply will help you decide whether this is a good game for you to play or not.
The game does not feature any intentional depictions of plural systems. I can totally see why some might interpret the characters that way, though. The exact nature and origin of the demon is intentionally ambiguous, and a bit of a spoiler, so I won't explain it here, but I certainly can if you'd like me to! I can say that I personally view the protagonist and the demon as completely separate entities from each other. Both have their own body and mind.
You asked about endings, and about how the demon character is treated. There are three endings. Some are sad, some happy. What all three endings have in common is that the protagonist never, ever sees the demon as evil. Other characters do, but the protagonist never does. I consider the two of them to be very much on the same team, as you put it, no matter what choices you make during the story. I should warn you that some of the endings do involve separation, and those paths were upsetting for me to write, so I imagine they may be upsetting to read, as well. But there IS a happy ending, and I have a feeling you'd find it easily.
I'm not sure how much more I can say without spoiling everything, but I want to reassure you that I find the demon character highly sympathetic and relatable, and I think that comes through in my writing. They are not an enemy.
Hope that helps! If there's anything else you'd like to know, please ask, I know my answers were quite vague since I was trying to avoid spoilers. And if you do end up playing the game, I'd love to hear your thoughts!