I know of three in this update, I edited my guide but I haven't tested everything, mostly deciphered the Regalia code:
Baela
-When and where you can get her: Mountain assignment "To Hunt A Paladin", but only after hitting camp level 5 of either Corruption or Marauding.
-How: Just crit To Hunt a Paladin after the right camp level. [EASY]
Donella
-When and where you can get her: "Stockades Survival", a special Great Plains assignment that comes up after you advance her plot (going to the Marshlands region) to the headpats/oral sex stage, and also it only shows up after you've done Reign of the Witch-Queen.
-How: Explore the Marshlands and follow the ravens several times, until you unlock the Marshlands Training assignment. Send many slavers to that assignment. After some 5 (?) slavers, visit Donella again a few more times so she tells you her story, and soon you get to the oral sex/headpats stage. After you do RotWQ, a special event will pop up after a slaver finishes Marshlands Training, opening her assignment. You have limited time to do it. But succeeding will get you Donella. [HARD, takes money and time to get that many slavers to do Marshlands Training]
Rizette
-When and where you can get her: Special Mountains assignment "Villa Most Vile". Here we go: 1) Get a True Angel. Yes, this is difficult enough on its own. Back in the day, the secret to getting one was the most asked question in this thread... the memories... but anyway, for that, get a critical success on Truly Divine with a former paladin of the Light/inquisitor in the first slot, then corruption, biomancy and a trainer. 2) Do a Holding Back the Dark assignment and get a DISASTER result. Even this has only a 50% chance of working, 75% if a Fel Tide event is happening. If this works, it will generate a Mistress of Torments assignment. 3) Success or critical on this MoT will generate Villa Most Vile.
-How: Remember that True Angel? You have to sell it on VMV and get a critical success. Good luck! [HARDEST]