The character, which won't necessarily be known to comics fans because the character (as far as we can tell) is original, is described via press release as:
"A sadistic gangster boss and nightclub owner, Fish Mooney's got the street smarts and almost extra-sensory abilities to read people like an open book. Imposing and hotheaded, she's not one to be crossed."
Pinkett-Smith will be working closely with Robin Lord Taylor, who is playing Oswald Cobblepot, a.k.a. the young version of the villain The Penguin. Taylor's character is a low-level gangster working for Mooney; and if we're to guess the arc, will probably work his way into Mooney's inner circle before ascending to the crown.
The show focuses on a young Commissioner Gordon (Benjamin McKenzie), starting with the murder of Bruce Wayne's parents and then moving forward to show the development of both Batman, and the villains who define him.
Also cast on the show, which has a series commitment for next season from FOX, are Donal Logue as Detective Harvey Bullock, Zabryna Guevara as Captain Essen, Erin Richards as Barbara Kean, and Sean Pertwee as Alfred Pennyworth. Bruno Heller ("The Mentalist," "Rome") wrote the pilot script and will serve as executive producer. Danny Cannon ("CSI," "Nikita") will direct and executive produce the pilot.
Stay tuned for more news on "Gotham" as it breaks.