The Gotham City Sirens come to Injustice 2 with Catwoman's introduction trailer. As Catwoman faces off with Harley Quinn, Harley asks her to join a new group called the "Gotham City Sirens," saying it'll be "you, me, Kate, and Babs!" - presumably referring to Kate Kane, a.k.a. Batwoman, and Barbara "Babs" Gordon, a.k.a. Batgirl.
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Injustice 2 will be available on Playstation 4, Xbox One, and mobile devices on May 16.