Loved the concept of this - a single-player co-op where the uncontrolled character is antagonistic against the other is pretty unique and clever! Though specific mechanics seem unique to this setting, the overarching concept could very well be adapted to other games and form a genre of its own.
The presentation of the GDD was pretty good, and I didn’t have trouble understanding the content. The structure seems a bit repetitive and not organized though - It might have helped to simply omit sections of the template that were not relevant to your game, and merge other overlapping sections for better clarity. I was also left a bit in the dark about the specifics of your actual core gameplay - what kinds of puzzles would exist, how would the players control and switch between characters, how exactly does combat work and what are the consequences, etc.
Also there are some sections where you seem vague or even undecided about what the game should be like - It’s usually better to pick a specific direction and stick with it (and include alternatives as footnotes if absolutely necessary) because otherwise I as the reader was pretty confused when trying to imagine the game.
The concept sounds fun to play and the puzzle-lover in me was already excited thinking of the kind of puzzles that the setting could offer. I liked the effort to make complementary strengths and weaknesses for both characters. The lack of specific gameplay details about puzzles/combat made it hard for me to say how fun it would be in practice, though. The overall story progression was interesting, as were the character backstories and details - but I’m not sure how the story elements are conveyed to the player in the game - is there dialogue, or written material, or something else?
Overall a great and very innovative concept - could use some more specifics about precise gameplay (and some diagrams!)