Yeah, this is a common theory in the fandom that I also personally believe. My guess is that it's something domestic, given the fact that children tend to adopt the behaviors of their parents. Bullies mostly just project their own insecurities onto others to feel better. Susie's disposition and behavior definitely reflects someone who's been through some tough times. Especially with how defensive she gets in situations where she feels even the slightest bit vulnerable.

Yes, exactly! Imho it's likely that her personal/family life is pretty messed up and unstable/unreliable on some very practical and tangible level, as shown with her reaction wrt her very own room in Ralsei's castle (especially the fridge), and I think her drinking the milk in the back alley is only so much played for laughs.
Moreover, the pleading look she gives Kris at the end of chapter 1 when asking them to go back to the Dark World the following day, I believe it gives us the first glimpse into her real self, a kind but traumatized soul that desperately seeks companionship but has convinced herself to be undeserving of affection, that ultimately found in Kris (or maybe in the person we have made Kris act out) the first friend she trusts and with whom she felt comfortable enough to open up to.
I dread the thought Toby might make us fight her in some future chapter, yet I fear it's a twist he'd be more than willing to pull...