we've seen that Susie's been a bully, in the world of "Deltarune"—I myself am uncertain how bad that problem is, or what's behind it.
it's interesting to me that her key emotional scenes in Chapter One of "Deltarune" are echoes or reflections of the harrowing scene at the start of the game, where she slams Kris against a locker because she's convinced they're going to get her expelled. she gives Lancer a taste of the same treatment—and gets a completely incongruous reaction to it! Lancer's happy! and that fits the general Through-the-Looking-Glass quality of "Deltarune" Chapter One, where everyone seems to be doing things for their own weird reasons.
and then later, the King of Spades gives Susie another reflected version of her bullying of Kris—and that really gets through to her.
interesting writing. I wonder if Fox was inspired by other media, here. we're not gamers, so we don't get game references, if any.
~Chara