She was featured and developed some in the Countdown prequel comic. In fact much of her relationship with Picard is well established there, and we see her at her best, then all that is pulled away from her. It ads well to her sense of betrayal and cover-up conspiracy paranoia when we first meet her in series.
And why Picard doesn't deny, but also doesn't surfer her wallowing or guilt trip, he just "needs" her to be back, to who and what he knows she is capable of. Knowing that's how they'll survive the rest.
I agree, (after Rios for me) of the new characters she would be my next choice, to get her own expansion Novel to flesh out more of her past.