You’re a vigilante/small time villain in Gotham. Batman catches you and finds out you’re only 15. He decides he’s going to help you even if you don’t really want it.
On your sixteenth birthday, you ended up trapped alone in a magical world. You end up becoming a knight and a magical hero in that world. Once everything is over, you are able to finally go home to Gotham. You use a spell to go home, you end up back in your bedroom in Wayne Manor. Within seconds, your entire family barges in ready for an intruder. They don’t expect to see you back after 3 years.
You are Bruce Wayne’s daughter. An alien life form kidnaps you and five other people to kill eachother for entertainment. Last survivor gets to escape. Six months later, you win.
You were Salazar Slytherin’s little sister. Legend said you died and the portrait in the slytherin common room is a portrait of you. Truth is, Salazar trapped you for centuries in that very painting. Then while Draco and his friends are messing around with magic on a late night, your painting was hit. You awake. You come to life and are freed from the painting.
You were a human turned deity. After thousands of years, you return to the mortal plane out of an old human curiosity. You end up getting the attention of Batman.
You were Bruce Wayne’s kid. A year ago, you sacrificed yourself for the world. Now no one knows you exist and you’re left alone with new powers building.
You were an experiment. Turn a human into a robot, an android. After two years, Batman stops the lab and scientists that took you, he sees a kid and decides to take you home and help.