Not bad. Just the Undertale fights give you a sort of break when it's your turn. And a slight hint at the next move. We also have no ability to attack or change the fight in our favour. So basically, the entire menu. We need that.
An introduction would be nice as well, get a little story in there. Dialogue for mid-game is a must though. Like when the boss gets damaged somehow (might need to clear up why it gets damaged as well)
The few bugs I could find are
-The movement glitch where you move faster when you stick against a wall.
- The hitbox for the red lasers or the heart itself is bigger than intended, as I lost lives without touching them.In the game, the hitbox for the heart is in the middle, not the entire heart itself.
- Once you get damaged, there's no immunity. If you get hit by the coins, you'll immediately lose 15 lives.
- The boss itself looks wonky on its own. Why does it have holes everywhere?
I'd like to see a debugged and improved version if possible. It already does feel like Undertale though