"Monster/alien/outsider in human society feels like they don't belong because they're #NotLikeUs" is pretty cliche, but reversing it and following a human protagonist helps it feel a little more unique and easy to relate to.
It also helps a lot that monsters are essentially just quirky humans so they're just as relatable, and that Kris's feelings of being different are pretty much entirely in their own head and nobody actually treats them any differently aside from some ignorant kid and that prick Snowy.

>>719732138
How are they opposites?
Alphys's Deltarune counterpart isn't Monster Mengele because she's a different person who has lived a different life. Maybe she's fundamentally the same in both timelines, but things have played out differently.
Kris could have turned out like Chara and Chara could have turned out like Kris, is that so hard to imagine? Do you think Toby Fox would write a story where a little kid with serious issues was just born irredeemably evil?

>>715470570
Kris was adopted at a very young age. If they were ever among other humans they don't remember it. Toriel and Asgore are Kris's parents.

>>714390843
Kris has an aversion to seeing other humans, Chara wanted to be a spree killer. Chara had a deep personal resentment against the humans in the village they came from, Kris is just uncomfortable with being reminded that they're different from everyone they've ever known.

>>713318439
We wouldn't have heard it from Susie, but the monsters in the apartment next door to her who ask Kris "Are you the human who lives at the top of town?" probably would have mentioned it

>>712743989
Retarded argument when monsters are clearly familiar with humans and have virtually identical gender roles