Curare

The most deadly of the League of Assasians, the mysterious Curare is dispatched to Gotham to kill D.A. Sam Young in order to prevent his testimony in an important case. Armed with incredible fighting skills and a sword that can cut through anything, Curare is an unstoppable force without a name or face. She has never failed to complete a job, but then again, she has never faced the Batman.

Many have guessed at a link from Curare to Ra's Al Ghul and Talia, but I think that she is more reminisent of Kyodai Ken , Bruce's old ninja enemy that he trained with. The action scenes with Curare were some of the best animated sequences in the show so far. That alone warrents her return. Not to mention that she is really cool. She is the Darth Maul of Batman Beyond ;-)