Hello Alamantus! I found your rulebook super interesting. As a person who has subjected their friends to too many overly-complicated RPG systems, I can certainly appreciate the simplicity (and yet robustness) of your design.
One thing about the GUTS qualities though, is that I would personally appreciate if you gave more examples of "traditional" skills in games for each of the characteristics, as for some unique qualities I feel like I do not really know where to place them. Also, perhaps a table of exemplary unique qualities would also be helpful? When discussing the system with some of my friends, they sometimes really did not know what quality to choose that complimented their backstory or the strengths in their qualities.
Also, like you said in the end of the book, I also think doubles is a super good tool for the game! I think I am going to implement a homebrew rule that double 6s is particularly special because that would be really cool :) I really love assisting mechanics, so I am glad it was introduced -- I have found explicit helping mechanics lends itself to character synergies, which is what I live for! I also love that they are based on both of the characters' qualities instead of a flat bonus because it simply makes sense.
Regarding the "essence" mechanic, I do not feel it is vague (like, at all?). I think you define very well the limitations of the essence mechanic -- and the diagrams are extremely helpful in this regard -- and then you give a wide number of combat/utility options for each example. Relatedly, I really like it! And its differentiation in low/middle/high tiers allows for its implementation in low-magic fantasy games without making them /too/ gamebreaking. Something I am perhaps missing is a mechanic that represents two/more essences affecting the same character or object (but not in a contest; ie. two PCs attacking the same bad guy). After all, since they are metaphysical representations of our energy tied to ourselves, isn't something bound to happen if too many of them get mixed around? Perhaps contrasting auras (eg. fast/slow) could have detrimental effects if they interact. I know you leave it up to GMs discretion, but I just thought they could add something interesting.
In regards to injuries, I must say I have not often played games with localized damage, so we will have to see how it goes. Always excited to try new stuff out.
I like the emphasis on GM control, even if I understand why you would be hesitant at giving so much of it. I think it interacts very well with the simplicity of the game, because it lends itself to a more narrative-driven experience that I personally find really enjoyable.
Oh, by the way, I love the status conditions table! I am /so/ bad at coming up with these types of descriptive adjectives (which are found in other games I have played) that I am probably going to use them there as well, haha.
Anyway, wasn't that a long comment. I just had a lot of thoughts after reading your book and I wanted to share them :) And thank you so much for sharing your game! <3