Very fun, I do enjoy the writing. Giving Kris a textbox speaking role instead his usual implied speech had me wary, but so far he's done well.
I do like how you treated the 'bosses' of each section, giving the option to spare someone wholly irredeemable like Porky makes for a good moral dilemma. Also, Fuck Porky.
Everybody is very well written, I personally appreciate Underfell being more about desperation instead of 'lul evuls'. Underfell Papyrus really carried the entire section.
Downsides... I personally had some minor visual bugs happening with the combat menus. too quick and the menu tears/strech across the screen or Susie/Noelle/Kris poking over the wrong HP bar. The game of UNO felt out of place for me? It's technically super impressive, but like, it's a long game to sit and play (if you're bad at UNO like I am) I know I spent 20 minutes staring at Susie as we go back and forth, never getting UNO because fucking Papyrus keeps calling out my uno while out of the game and I'm too quick on the draw to let Susie get UNO. Also, I get the combat UI is supposed to be taking elements from all three games, however UNDERTALE's ugly targeting reticle is still ugly and makes the perfect timing window unclear. Especially with three people swinging at once.