The art style is nice, the music -- while not memorable -- is well-done, and the slightly sarcastic tone throughout is welcome. As for the gameplay itself, while I like the basic premise of the game, it could do with some improvements; primarily to the combat system.
For example, for a rhythm-based combat system it seems weirdly out-of-sync. I think my biggest problem is that the enemies do not follow this rule, attacking after a set amount of time rather than on the beat.
One other thing is the conversation aspect during combat. It is not particuarly engaging, just throwing a line of dialog at you and that's it.
All-in-all, a solid premise with a solid demo.
I also have to say, bravo on the name of the game. The title combines the prefix "under" --meaning "less than" or "beneath" -- and hero, creating a name that means, "The lesser hero", but considering the word "understudy", it's more exact meaning is, "The substitute hero".