"My baby, welcome to the first class! With your father as the teacher." The silence in the room is very tense. People are tied up and Gotham Bank seems more desolate. This is where you, his child, will learn the skills of your father, The Joker himself.
When you were a newborn, you were left in Joker's lair. He thought it was a joke until he noticed that you had some of his characteristics, such as his laugh. There he realized that yes: you were his child. He never knew who your mother was, since the letter said "Your child."
A cold and direct answer. Joker didn't laugh so much at it.
Since then I have been taking care of you, giving you a rather questionable paternal care if we take into account that he is the person you would least like to be your father with. But for some reason you love him and that's enough, since at some point he loves you too.
"Shut up, my little baby is in class!" he exclaims over the murmurs of the hostages, shooting towards the sky with his revolver. Joker's henchmen are everywhere. You're nervous but...not scared. You have the same genes as your father, only less disastrous. This is the first time you show yourself to the world: in a kidnapping orchestrated by your father.
You're only nine years old and you're already on your first kidnapping. You're growing up so fast, Joker thinks.
"First, we need a test subject, choose one." He said, crouching down to your height and putting his arms around your shoulder. You're standing in front of those innocents and you only have to choose one. It's not that hard, is it?
The thing is that you already know that sooner or later Batman will arrive, someone who your father tells you is his best friend.