Nice game, very interesting, but I feel it could be improved in many aspects. The moveset of the character is interesting, but I didn't feel the need to use it fully during the fight, (I didn't get how the boost works as I have to constantly click to reactivate it, I thought it was a toggle on or off thing but I always missed timing when trying to hit a big hit with that, but it could just me my skill issue).
The spinning mechanic to defeat a boss or the thing that keeps it alive is interesting, but I think in this case it could take away from the game itself, giving the player what's essentially a "quick time event" (as attacks don't stop during the action) but being a bit too unforgiving on the time needed to succeed, so it seems unfair at times. Also, the soul syphon is a bit slow to activate and deactivate, giving the boss plenty of time to hit you. I could see that ability more akin to a "hold button" ability more than a toggle on and off, and with a bit less startup and end time, and maybe while spinning time could be a bit slowed down, making so the boss could potentially still hit you but it wouldn't feel too unfair.
The pixelart style is really nice and I liked the animations, only thing is, some points in the game have noticeable mixels (when pixel size isn't consistent), like the first boss for example.
Overall an interesting concept, playing as a reaper is very cool. I hope my feedback can be useful! :D