and

We never hear from anyone in deltarune that Susie bullied them.

No one, except Kris.

And yet

there are hints that Kris might have liked Susie even before her character arc.

It gets supported by how well they get along after it. Kris isn't holding a grudge, they embrace this new friendship with open arms, as if it's something they've been wanting for a while.

But why? Why would Kris like someone who hurt them?

Exactly why Lilo liked Stitch when he did nothing except make her life more difficult. She saw herself in him.

Kris understood Susie the way no one else did. They knew why she lashed out, it's the same reason why they kept pulling pranks and acting out. They felt like an outsider so they might as well play the part.

When they met Susie, they saw someone who was just like them. And they weren't wrong, because ever since they became friends, they've practically been sharing a braincell

this does not. they're playing world of tanks which is like the "call of duty" of tank games (casual, players only slightly bad-smelling). it also doesn't have support for tank peripherals. no game does. no trainers do afaik. which means that (assuming this isn't just a video editing) all of that shit they are fucking with translates into mouse/keyboard inputs that the game understands. that's weird/hard and perplexing, uh, and considering that "tank peripherals" aren't a thing that exist i can only guess they built them theirselves

But that's the thing. She doesn't exaggerate or grotesqueify or alter people's bodies to fit some standard. (Except insofar as she draws different species differently, and those are exquisitely practiced to ensure they have the same diversity of appearances that humans do.) She just presents people exactly as they are, complexities and oddities and all.