EV-9D9 was originally a V series supervisor droid who worked on Cloud City; but her programming degraded and she became obsessed with torture.
After she was banished from Cloud City, she somehow made it to Tatooine. There, she was hired by Jabba the Hutt as supervisor of the palace droids. She lived in a dungeon chamber with her assistant 8D8. From there; she worked palace droids until they broke down, developing twisted methods of droid torture for both her amusement and for increasing motivation.
Her actual role in the movie was barely significant. EV-9D9 appears once in the movie. R2 D2 and C3PO are taken to her droid torture chamber in the bowels of Jabba's Palace to be sorted. She assigns 3PO as Jabba's new interpreter (after pointing out that the Hutt had lost his temper with his last protocol droid and disintegrated him). She has two Gamorean Guards take him up to Jabba's main audience chamber. She then assigns R2 as a waiter on Jabba's sail barge.
After Jabba died; the B'omarr Monks of the palace dungeons either killed her or drove her out of the palace along with the rest of its nasty inhabitants.
EV-9D9 was a V series supervisor droid. Her body was a legless torso that could lock on to different sets of legs and wheels. She is brownish red. She has one short arm and one long but skinny arm. She has a semi-humanoid head and a speech mimic flap that moves when she talks. She originally had two eyes, but added a third after she lost her mind.
EV-9D9 tragically lost her mind due to degradation of her CPU's programming. After this; she devolved into a sick twisted evil droid, and developed a love of pain. After being fired from Cloud City for inflicting torture on the droid she supervised; EV-9D9 inflicted torture on herself to satisfy the urge, even adding a third eye next to her left for the sake of the pain. Upon securing the job as Jabba's droid supervisor, she was once again able to torture droids for fun. In spite of being insane, EV-9D9 still retains her strict and efficient supervisor nature. That combined with her twisted love of torture, makes her a cruel and evil official as well as Jabba's idea of the perfect droid supervisor (so much so that he is actually willing to pay her).