With Cassie you have to do her route and the "thug girl" do you mean that thug mother? Well there's nothing you can do with her. or if you mean Alissa that other thug girl you need to complete Cassie route
Well Alissa route isnt finished yet but you can do some process till it yet available, after to make her Cassie "pet" you have to sneek in the abandoned city till you found her as an enemige (you don't gonna fight her), the rest figured out yourself I don't gonna spoiler you, if you stuck on one part you can reply me.
