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’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.
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 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 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.