I think Dess is fully manipulating and using Kris so she can forcibly insert herself into the Prophecy and be "special".
>The random sport equipment and instruments Dess has all over her room and in her closet
>She likely was trying to find something she was easily good at or didn't have to work towards, giving up when something inevitably goes wrong
>Dess finds out about the prophecy somehow and becomes obsessed with being one of the "heroes" but learns she isn't one of them
>Kris on the other hand COULD be one simply because they're a human
>Out of jealousy, she becomes much more harsh towards Kris over trivial reasons and becomes obsessed with their soul, collecting the pile of hearts in her closet and sneakers she can't even wear
>Eventually she finds out more about the prophecy and possibly how Dark Worlds work and realizes she can manipulate Kris to use their soul
>Dess is in the perfect position for this since she's older, from the mayor's family, rich, and the two families being friends
>Asriel becoming her boyfriend could have been another way to get closer to Kris
>She makes a promise to Kris that if they help her get what she wants, she'll make them "normal" somehow, likely by turning them into a monster since Kris has worn the headband for a while by now
>She could even fake showing them how this is possible to secure Kris's "promise"
>The Knight's general way of acting "superior" when they do anything, like the stairs posing
Undertale has Flowey and Chara manipulating you (or each other), so it's not exactly 'new'. It also makes Snowgrave into an even more fucked up route since now you're forcing Kris to manipulate Noelle, perpetuating a cycle they don't want to continue.
If the game chapters released more fully together, it's possible that the main route would've been finding out Dess's manipulation over Kris (and YOU in a meta way) and Snowgrave would've been a "revenge" route that forces Kris along for the ride.

then again it's just a theory :)