Although I don't really like her character much (largely because of her caustic persona and her standing between Cal and Gillian at times) I have to say I find Zoe interesting.
For starters, she is extremely career-driven, just as much as Cal and Gillian. And, from what we've seen, I think that like Cal and Gillian, she's centered upon her work being for the greater good, i.e. justice, closure, etc.
Initially, I believed that Zoe wouldn't have liked Gillian even if Cal didn't exist, simply because she is a sharp kind of person, who undoubtedly views Gillian as too soft and a do-gooder. That may be true, but now I tend to think that while she's stood between Cal and Gillian to a degree, a lot of her behavior is due to her seeing that special closeness the two have, which she herself didn't have even though she and Cal were married for many years. If you recall in the episode where we saw flashbacks of when Cal and Gillian first met, Cal opened up to her almost immediately, revealing a number of things he'd never told to Zoe. I suspect Zoe is very aware of this.
It's too bad we never got to see Zoe and the infamous Roger, as I think it would've been really interesting and reveal a lot about her character to see what kind of man she'd attract, and how she interacted with someone outside our regular circle of characters.