Harley Quinn originally worked as a psychiatrist at Arkham Asylum. When the Joker was imprisoned there by Batman, the young psychiatrist took a particular interest in him, regularly visiting and speaking with him. Their frequent visits led to Harleen being gradually psychologically sabotaged and manipulated by the psychopath, eventually falling in love with him however, the Joker sadistically subjected Harley  to shock therapy which had a severe impact on her mental state.