Nice demo! I really like all the attention to detail you've included, like naming yourself gaster crashing the game, and such. I'm excited to see what you've got in store for the future!
Anyway so here's a few tiny bugs(i think) I found:
- During the game over screen, the gradient background is visible along the edges of the screen
- Holding "c" at the start doesn't seem to work sometimes(could just be me on this one)
- Any text after the first line is slightly off to the right
- Moving the cursor in the ACT menu lets you select options that don't exist(ignore if you plan on adding more ACT options later)
- Holding "Z" after any Sans text box finishes lets you stall the game until "Z" is released
- While the soul is moving up, rapidly pressing the arrow keys in any sub-menu(ACT, ITEM, etc.) lets you move the soul out of the menu area(only sometimes works for me)
- Holding "Z" after Sans' attack ends causes you attack instantly, bugging out the menu and soul
- You can press "X" right after selecting MERCY to prevent the damage thing from happening
- Holding both left+right or up+down allows you to cheese the orange bone attacks