Baldur's Gate II: Enhanced Edition

Complete the quest "Save Mazzy's Sister, Pala" - Recruit Mazzy to the party either during or after completing the quest "Investigate the deaths in the Umar Hills". After some in-game time with the party, her quest will unlock following a scripted scene. Make certain to complete the first task in the quest - Mazzy will leave if you fail to do so within a reasonable amount of time, making the achievement missable.