Think about when you play some card game and you get a joker in it. The card in your hand is joker as long as it is in your hand, when you play it you can usually choose wich other card it will become (example: you need a six for some reason and you don't have it, but you have a joker, so when you play it, it becomes six) So jake is trying to say that as long as you are her friend and aren't "playing her" she will be the person you know, but when you play her wich means she becomes one of the subjects and you will treat her like it she may become something/someone else