HK-57 was a type of assassin droid from the HK-series. It was manufactured within a droid factory that formerly belonged to the Separatist faction. This factory was located on Mustafar and was overseen by the renegade assassin droid HK-47. After the discovery of defects in both the HK-57 and the following HK-67 models, HK-47 began development of an updated model known as the HK-77 droid.
HK-47 mentions this particular droid model in a recorded message. This message can be found on a datapad labeled "Droid Factory Records." Players can acquire the datapad during the missions leading up to their encounter with HK-47 in the Battle of Koseyet, within the Star Wars Galaxies expansion pack Trials of Obi-Wan.