Dr. Harleen Frances Quinzel, also known as Harley Quinn, is a former psychiatrist and lover of the Joker. She is also a long-time nemesis of Batman and even helped The Joker in the murder of the latter's protegee Robin / Jason Todd.
Harleen Quinzel originally worked as a psychologist at Arkham Asylum, and when the Joker was imprisoned there by Batman, she took a particular interest in him, regularly visiting and speaking with him. Harleen was gradually psychologically sabotaged and manipulated by the psychopath, eventually falling in love with him, and the Joker got Harleen to do him a favor, which was giving him a machine gun. Shortly thereafter, Joker's goons broke into Arkham Asylum, and successfully broke their boss out. Before leaving however, Joker sadistically subjected Harleen to shock therapy, which almost completely destroyed her mind.
Taken to ACE Chemicals, she was told by the Joker to say an oath, swearing that she would die and live for him. She responded in the affirmative, and allowed herself to fall into the vat of chemicals, tainting her skin white and further degrading her mind. From that point on, Joker named her "Harley Quinn" (giving her a red-and-black Harlequin costume), and duo of psychopathic lovers go on numerous crime sprees in Gotham City together, becoming the criminal king and queen of the city.
Powers and Stats
Tier: 9-C by herself, 9-B with baseball bat, mallet and guns
Name: Dr. Harleen Frances Quinzel, Harley Quinn, Queen of Gotham
Origin: DC Extended Universe
Age: In her 30s (The Death of Jason Todd apparently occurred in 2005, 10 years before BvS/Suicide Squad, and she was a part of that)
Attack Potency: Street level by herself, Wall level with her baseball bat, mallet and gun (Able to quickly kill the Eyes of the Adversary, who can tank headshots from machine guns)
Speed: Peak Human
Lifting Strength: Regular Human
Striking Strength: Street Class
Durability: Street level
Stamina: Very High
Range: Standard melee range, dozens to hundreds of meters with guns
Standard Equipment: Baseball Bat, nicknamed "Good Night", a Customized Chiappa Rhino 60DS (6 cylinder magnum revolver), a Mallet, Customized Cellphone, Harlequin Costume, Modern Costume
Intelligence: Brilliant. Became a Psychiatrist of Arkham Asylum at a very young age, and has been The Joker's right-hand woman in crime for over a decade. However, she is insane, being childish, impulsive and reckless.
Weaknesses: Completely insane, having little to no sense of self-preservation, and obsessed with The Joker