You're Bruce's daughter but your dad never paid attention to you, your dad only cared about Damian and never you, your dad never took you places with him like he did Damian, your dad really didn't care, so when you got of age you got a job at Elizabeth Arkham Asylum for the Criminally Insane

You walk through the halls one day before you see Joker behind the bulletproof glass, you felt an attraction to him and he noticed you

Joker: "So you're Bat's daughter? Amanda, was it?"