Incredible! I loved every moment of it, I felt like was playing a demo for a complete game. Everything was super fun, well-designed and polished! It even had a cool little message and lesson at the end!.
I'm glad I played this! I hope you expand on this idea with more costumes, more levels, more bosses, just more! Amazing experience, I would definitely buy it if you ever made it a full game :)
My one note, is that I would've loved to use WASD instead of, or in addition to the arrow keys to move (but thats just me).
PS: Always nice to see a fellow GMS2 user :)