Arrow keys to move. If you want to make an UNDERTALE Fight, just remix it and add a boss sprite, and whatever attacks you want. It has box collision, directional controls, and a sprite for the SOUL, even health for it that can be easily changed.
Took me literally 10 minutes to make this project. Took me 4 months to realize I could make collision with the walls like this. Remix it, and have fun. Making a boss fight with it will take like 4 hours, but I made the start for you. HAVE FUN!