Isis is Catwoman's pet.
Catwoman is shown to keep many cats, among those is Isis.
Events in Injustice
Isis is shown during Catwoman's intro, taking a diamond from her and bringing it to the thief's lair. She is also playable in a S.T.A.R. Labs Mission, during which she has to sneak into Gotham City's Museum to collect data on the location of Batman's hidden lair.