They're in the Dust Reaper HQ, but it's incredibly easy to miss her. She doesn't have any unique 'barks' to point herself out (unlike most unique bounties) and she usually gets buried in the swarm of other Reapers if you're attacking the tower head-on. If you're attacking the tower rather than going in stealthily, don't be surprised if you literally have to sift through a pile of dead Dust Reapers to find her.

She'll be wearing blackened chainmail, an assassin's rags, and a turban. For recruiting her, just talk to her while she's in any cage and ask her to join you (you can actually throw her into the cages in the ruined Holy Nation outpost where you find her).

The problem I had with finding her was that when she spawned, she would immediately go into "Home territory aggression" mode. This, coupled with her being fast AF, meant that she would haul ass out of the ruined outpost, only to become a proverbial needle in a haystack to locate -somewhere- in the Border Zone (and maybe she even dipped into the Swamp).

Parking my squad directly in front of the watchtower at the ruins - and then importing my game - was the only chance I had at catching her.