It just feels weird that she has a complete character 180 that quickly.
Deltarune is fundamentally about hurt people and the coping mechanisms they use.
Susie copes with her harsh home life (it's implied to be abusive, but we don't know) by being a bully. Often times the kids at school who beat other people up learned it from their parents or siblings. It's both learned behavior and a defensive mechanism. With Susie in particular there's various times when it becomes apparent that she has trust issues as well. Many people like this do want to be nice and get along with others, they simply don't know how, or find that pushing everyone away is easier than risking being hurt. She goes with Lancer because he tells her that he looks up to her. She's desperate for connection but has a hard time opening up to it. Susie's harsh exterior is a facade that she continues to struggle to drop entirely at time, but it's really not that surprising that once she feels comfortable around Kris she drops the facade.
Kris copes with his trauma by disassociating, (while I don't agree with the aggressively assigned label the fan-base slaps on him of being non-binary, it is clear that he has identity issues, and if you look into it, you'll find that a lot of people that have gender issues often have it stem from trauma.) and by allowing those around him to make choices on his behalf. He doesn't like being observed or seen, as evidenced by the issues he has around playing piano around others. Susie seems to be the only one that makes Kris feel comfortable being themselves, which is why he only relents and plays when she begs him to.
Ralsei copes with their situation (which we only get implications about at this time) by throwing themselves at their duty and attempting not to have an identity at all. Even going as far to call other darkners who do try to have an identity selfish.
Pretty much all the other kids have similar issues and coping mechanisms. Some don't have it as bad, like Catti, who is only unhappy with her family because she isn't like them, but no one in this town seems to have a "good" family life.
I could go on, but I think you get the point, when I was young I hung out with the "freaks" myself, and looking back on it, everyone who was a social outcast was so because of abuse they had suffered. Kids do not know how to cope in healthy ways. In my opinion, that's one of the main themes of the game.
I actually find it pretty impressive that Toby can write about stuff like identity without fagging it all up.
