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Hi

Passionate at her best, and emotionally volatile at worst, Lisa never does anything halfway, and that includes how deeply she feels each and every emotion.

I think I understand the meaning of "at her best" and "at worst", but I'm still not quite sure what the author wanted to say in that bolded part of the sentence?

EDIT: Or maybe it means in the best case she was impulsive and in the worst case she was emotionally unstable?
Comments 
You could read best and worst as "on a good day" and "on a bad day", or "when she's in a good mood/bad mood".

Does that help?