"What is it with all the insect themed female villains in Gotham lately?" -Stephanie Brown, Robin #127
Not to be confused with the thing embedded in Jaime Reyes's spine.
Scarab was a female assassin in a high tech suit hired by Johnny Warlock to kill Tim . She put Tim's face through facial recognition software, found nine kids who matched it in Gotham and started to kill each and every one. Before she got to the real Tim, Batman and Robin (Stephanie!) caught up to her and Steph put a tracer on her.
Batman went to fight Scarab and got blinded. Against his orders, Stephanie ran in to help and got captured by Scarab. Batman was forced to let Scarab go to save Steph's life.
He then fired Steph as Robin.
Like most comic book assassins, Scarab claims she is not a mindless killer, and her code of ethics requires her to only kill people she's paid to kill. She has a high tech suit, but is plenty dangerous without it, as Stephanie found out.
Later, Stephanie hired Scarab to help her and Tim reclaim Gotham, without Tim's knowledge. She told Scarab to use non-lethal weapons, but the mysterious Red Robin also decided to hire Scarab for his own means and likely paid her more, because she disobeyed Stephanie and would have killed Tim if Stephanie hadn't tackled her. Stephanie hit Scarab a few times, and tackled her again when she launched an RPG at Tim, making her miss, but Scarab eventually flew away and was still working for Red Robin.
Later, Scarab returned to Gotham to assassinate Tim AGAIN. Tim needed information about the Scarab, so he visited Steph in the Batcave. Steph asked him why he wants to bring up the whole Scarab mess again after all she's done to put that behind them, and he let her know it's noting like that, and he just needs to know what she knows for a plan he's hatching. She somersaulted off the high bars, saying if Scarab's in town, Steph wants a peice of her. Tim informed her that if his plan fails, he'll be dead, and Scarab will be all hers. Steph snapped "That's not funny" and Tim replied"who said I was joking?" They pooled information, both deciding the villain is Egyptin and Steph lets Tim know about Scarab's Middle East contacts so he can track the woman down.