(Played through steam)
An incredibly promising demo! I thoroughly enjoyed the combat, despite not being the greatest at rhythm based games. The undertale references are good, as this game almost appears inspired by it. However, I hope it does not become known for it being like undertale, as that would take the spotlight away from an amazingly creative piece of work.
Overall, my only suggestion would be to change the menu screen font, away form the one undertale is commonly known for.
This is a Great game!