I didn't realize what stacking did until a few turns in, but it seemed like an interesting idea. I always like when people take a classic game and add a rule change or two to really shake it up. It is tough to see how much it would change or if the change is good without playing it a bunch but it's a neat idea for sure.
I liked the characters reacting to taking pieces, that adds some personality. I might keep them on the board at all times though, that way you could have them react to each move and also to who was winning overall.
I also think that making the board taller and/or adding bouncing off the wall if you had more than one movement might be cool additions.
Anyway great job and congrats on finishing!