Cassandra is the first underboss you can recruit in Mafia III. To unlock her as an underboss and get access to her associates and connections, all you have to do is progress through the story normally until you reach the mission “Old Times’ Sake.” This will prompt you to track down the Haitians, and interrogate an informant for Cassandra’s whereabouts. This should be pretty simple; just wait till the informant is isolated from the other two guards, then take him down with a melee attack, and the interrogation should trigger.
After that, follow your waypoint and that should lead you to Cassandra’s house where another cutscene will trigger, and Lincoln will convince her to join his ranks. At the end of the cutscene, you’ll have completed the mission, and you’ll now have access to Cassandra’s associates and her weapon cache. If you pull up your map, you should see an icon indicating her weapon cache location, and you’ll be able to head there to stock up on your gear.
Cassandra will also be able to unlock other helpful perks throughout the course of the game, as long as you make sure to keep her happy.
Mafia III is now available on PS4, Xbox One, and PC.