This is an absolute gem of a fangame. Even setting aside the main conceit (dating Sans) it's a wonderful love letter to Undertale in and of itself! It manages to pack so much into its runtime, and I find myself replaying it every so often because it has the same cozy feeling I loved from Undertale. Even more than keeping the mechanics Undertale initially employed, it's important to nail the heart and the feeling behind the game, and you really got everyone here 100%!
!!! MINOR SPOILERS HERE, BETTER SAFE THAN SORRY !!!
Seeing and interacting with the other characters feels like coming home, a happy little glimpse into their lives past Undertale's end. I especially adore the roleplaying scene with Alphys and Undyne; it's hilarious while also feeling warm and affectionate, with all the love going around everyone, between the MC's pining and Undyne and Alphys's own relationship! Every character was incredibly true to themselves, up to all their quirks and mannerisms, and it makes every part of this game such a delight to read.
I can't skip over the writing of the main draw either. The way the relationship with Sans is built over multiple RESETs is so charming and feels so true, with the hints of his character we got in the original game; of course he wouldn't be that easy of a nut to crack, even with his friendly attitude. More than that, the moments between him and the MC are written with such genuine emotion and care that it never fails to get me, no matter how many times I replay the game. Catch me in the club crying over the quiet wonder perfectly captured in the scene where he sees MC's soul even after I've read it for the 100th time in a row!!
From the bottom of my heart, thank you for bringing this project to life and releasing it into the world.