Really like the game so far :) The creative new approach to game mechanics is really nice!
My ideas for improvement:
1. Clearer visual indicator for lost hearts. Perhaps make the area around "broken" hearts black instead of white? This would make it easier to recognize from a glance. I was often not sure about how many hearts I had left without looking at it focused.
2. I missed a way to regenerate health. Perhaps the enemies could drop health or you regenerate some by hitting them(/alternative way?). Otherwise they could drop currency. As it is now I felt the combat using the first ghost took to long (3 hits with long cooldown in between for one small enemy) and lacked a purpose, so I evaded enemies instead of fighting them.
Will update with other stuff, that crosses my mind :) Hope there is something helpful in there for you ;)