Deadly adversary or just another of The Joker's victims?
An Arkham Asylum psychiatrist assigned to treat The Joker, Dr. Harleen Quinzel instead became obsessively fixated on her patient, believing herself to be in love with him. She helped him escape confinement and took on her own criminal identity as Harley Quinn.
Quinn is a violent and unpredictable felon whose only motivation, beyond general mayhem, is achieving The Joker’s approval. Because of his cruel and mercurial nature, this in some ways makes her just another of his victims—albeit a very dangerous one.