Detective Ellen Yin is as physically adept as any male cop she knows. Ellen may be new to the Gotham beat, but she's no rookie. Previously head of her department in Metropolis, she transferred to Gotham looking for new challenges. She’s a career cop at the top of her game, true blue and by the book. Not warming to Bruce Wayne so easily, Ellen doesn't have it out personally for The Batman.

Yin became Bruce's Friend from the End of Season One to the End of Season 2, where she no longer appeared in the spotlight. Yin became the Dark Knight's partner in fighting crime and figuring things out. To say...Batman's inside informant.

Yin no longer appeared after Season 2, though it is implied she becomes Police Commissioner in the near future during the fourth season episode "Artifacts." She is voiced by Ming-Na.

Info from Wikipedia and myself