Gotham City Sirens refers to the informal cooperation that exists between the primary female villains of Gotham City: Catwoman, Harley Quinn and Poison Ivy. They started working together after Batman's apparent demise in Final Crisis. Though the name is used on comics and in story titles, the villains do not use it as a team name in the stories themselves.
The concept was first explored by Paul Dini and Guillem March for Gotham City Sirens #1 (2009).
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