HK-51, a unit from the HK-51 series assassin droid line, was created by the Czerka Corporation in the time leading up to the conclusion of the Great Galactic War.
Near the end of the Great Galactic War, the Sith Empire placed an order with the Czerka Corporation for a shipment of HK-51 series assassin droids. However, the Sith warship named Fatality, which was transporting the droids, was taken over and subsequently crashed on Belsavis within the Frozen Lake region of Section X. The droids remained inactive for several decades until, during the revitalized Galactic War in 3639 BBY, an important figure visited the wreckage. They discovered that one of the HK-51 units could still be salvaged and obtained the necessary components to repair it. Upon being reactivated, the droid greeted this figure as its new master. While in service to its new master, HK-51 began to notice a rapid decline in its combat and assassination effectiveness. An analysis revealed that this trend would eventually render the droid non-functional. The root cause was identified as a failsafe subroutine embedded deep within HK-51's programming. This failsafe would trigger a complete shutdown unless HK-51 successfully assassinated at least one individual from an implanted kill list, though killing a target would slow the degradation of his protocols. Consequently, its master tasked HK-51 with assassinating one of the targets. After successfully completing the mission, HK-51 reported that its protocols were functioning at peak efficiency, as if it had just been manufactured. Developing a strong admiration, almost bordering on idolization, for its master, HK-51 grew concerned about the possibility of being reprogrammed to turn against the master it respected. Seeking a solution, HK-51 discovered a loyalty subroutine that could permanently bind its allegiance to its master and anyone designated by the master. The master agreed to activate the subroutine, much to HK-51's delight, who then suggested celebrating by eliminating a large number of enemies.
Even though his master vanished during the Eternal Empire conquest in 3636 BBY, HK-51 was reunited with them in 3631 BBY after his master was rescued from imprisonment on Zakuul and became the Commander of the Alliance that stood against the Eternal Empire. HK-51 met his master again at the Alliance base located on Odessen. During a gathering at the Alliance cantina, the droid was mistaken for HK-55, another HK-series assassin droid that Tora had recently encountered on the Asylum.
HK-51 was engineered to be an extremely effective assassin, deriving immense satisfaction and pride from its duties. Unlike its predecessors, it was programmed to be exceptionally polite, cheerful, and amicable towards its master and their allies. The programmers believed that earlier models were too disturbing due to their brutality. HK-51 itself even described models such as HK-47 as "unstable." HK-51 did not seem to exhibit the hatred or disgust for organic beings or other droids that was typical of previous HK models. HK-51 also showed a dislike for political maneuvering and internal conflicts within factions, although it would engage in such activities if ordered to by its master. On the surface, HK-51 appeared to be nothing more than an outdated Systech Corporation protocol droid. This was common for most HK-series droids and a key factor in their unparalleled lethality.
While it was capable of both precise assassinations and large-scale killings, HK-51 preferred assassinating single targets that were difficult to locate. In this aspect, it was more similar to HK-47 than to its direct predecessors, the HK-50 droids. Like HK-47, HK-51 considered its assassinations to be an art form. However, it was programmed to prioritize efficiency over artistry, although it enjoyed it most when it could achieve both. HK-51 had a fondness for organics, as its negotiation/protocol programming was more pronounced than in earlier models. It proudly claimed to be an excellent conversationalist and frequently used code words to conceal its true nature and purpose from those who might be alarmed. It often referred to its targets as "undesirables" or "troublemakers," assassination/murder as "liquidation" or "deletion," and those who would be alarmed by its true purpose as "polite company." HK-51 also frequently mentioned a "Maker," seemingly a deity responsible for fate.
HK-51 incorporated several improvements and upgrades over its predecessor models. In addition to being more armored and accurate, HK-51 was significantly more mobile and maneuverable. It could perform faster and smoother movements, almost mimicking those of organic humanoids, and execute leaps and combat rolls. HK-51 also had integrated weapons and equipment: wrist-mounted micro-missile launchers and railgun, hidden smoke grenades, a stealth field generator, and the ability to self-repair if critically damaged. As a last resort, HK-51 was equipped with a powerful explosive that it could detonate in a kamikaze attack to complete its mission.
In Star Wars: The Old Republic, HK-51 is featured as a companion character accessible to all classes. Unlike other companions, acquiring HK-51 requires completing a specific questline that starts in the Section X zone of Belsavis and leads to the discovery of the crashed Imperial battlecruiser Fatality, which carries a full contingent of damaged but salvageable HK-51s. To claim an HK-51, players must explore the abandoned Czerka Corporation cruiser Theoretika in the Unknown Regions and obtain a set of HK schematics. Following this, they must gather specific HK parts from Coruscant, Dromund Kaas, Taris, Hoth, and Tatooine, as well as the False Emperor and either the Maelstrom Prison (Republic) or the Foundry (Empire) flashpoints. Once the parts are collected, players must return to the Fatality wreck on Belsavis and defeat a Sith of the Dread Host known as "The Lord of Agony," after which they can claim their HK-51.
Players of both factions obtain slightly different versions of HK-51, with unique conversations and ambient dialogue, along with a list of three targets, one of whom the player must choose for HK-51 to eliminate. However, during the Knights of the Fallen Empire expansion, the original eight class characters disappear, and HK-51 appears as the Outlander's droid, provided the player unlocked him before the expansion. HK-51 provides environmental commentaries only on Makeb and participates in the cutscene between the player and HK-47 during the "False Emperor" flashpoint, where HK-51 asserts that HK-47 is an inferior model, while HK-47 maintains that he remains the pinnacle of HK droid models.