Okay, yeah, I wasn't thinking about of negative trait, but rather of overall personality.
Actually, thinking about it more, emotional detachment (Excluding her few friends which are very close) seems to work for her.
Seth, on the other hand, would probably be the foil of Alice: A very outgoing, emotional individual, who is known to be quite helpful, sometimes to the point of self sacrifice.
(Not saying Alice isn't helpful, It's just there to foil this next point.)
The main point of differentiation is that Alice ends up thinking before doing something (maybe too much?), Seth is a spontaneous individual, often not looking before leaping. He is often quick on his feet to compensate, but nonetheless he gets into trouble, and when dealing with all those pan-dimensional affairs, being quick isn't always enough. However, he ends up being good at cracking jokes for that exact reason.
The personatities don't really switch that much until The White Queen incident, as Alice becomes a queen and become a very emotional individual, with little control over them (Angst Nuke turned Good Is Not Soft with a bit of a Hair-Trigger Temper), while Seth becomes 'Scarecrow', he forces himself to think more about his decisions and actions, albeit still with that spontaneous nature which he struggle to control, leading him to seem irate most of the time out of self-loathing that he can't defy his nature(I'm not actually that familiar with TV tropes, but Rage Against The Reflection will probably happen). After finding each other again, they end up starting to balance each other out, albeit not fully to their previous states of mind, of course.