Undertale changed me from a completionist into someone who only does things in games when I enjoy them. I used to go for all the achievements, no matter how tedious or frustrating. But after Undertale, I came to realize, as the game tells you, that just because you can do everything possible in a game, it doesn't mean that you have to. I used to be really bothered if I didn't have that 100% on my save files for pretty much every game I ever played. But after seeing Undertale (yeah, I only ever watched people play it, never played it myself other than the demo, I was too unsettled by the game's art style and overall tone to want to be alone with it XD) I became totally okay with having an "incomplete" save file in a game.
It's very freeing. I don't feel "forced" to, say, catch several thousand fish in Animal Crossing just to get a stamp. There was a point in time when I would have felt like I "had to." But Undertale made me realize that I never "have to." And that's a really big deal for me. I can feel done with a game even if my save file says 99%, or 85%, or 50%, or whatever percent when I feel like I've done everything I wanted to. If I'm given multiple choices for how to treat a character in a video game, I used to go back and try them all, because I "had to" see everything the game had to offer. But now, if I would feel bad for picking certain choices offered to me in a game, I just don't choose them.
Overall, Undertale makes me uncomfortable. There are some pretty dark, unsettling, disturbing parts in the game, and I'll readily admit that I tabbed away from the videos I was watching when scary things appeared, just listening to the Let's Players crying out, "AAAAH, WHAT IS THAT?!" and feeling glad that I wasn't looking more closely. But I freaking love Papyrus. And the music in Undertale is a gosh darn masterpiece. I never played or bought the game, but I bought the soundtrack, because that's just how good the music is.
As for Deltarune, I didn't play that, either, though I watched a video of someone playing the first part, and, like Undertale, I found it too unsettling to want to play it myself. But, unlike Undertale, none of the characters endeared themselves to me, and I wasn't curious enough to find out what happened next, because it was actually pretty disturbing to me.
Wow, that was a lot more than I was expecting to say. I hope you enjoyed my wall of text. ^o^;>