HK-55

In 3631 BBY, HK-55 assisted his masters Lana Beniko and Koth Vortena to rescue the Commander from Arcann's carbonite prison on Zakuul. HK remained with Koth in their escape shuttle while Lana and T7-O1 infiltrated the prison. However, their prison break didn't go unnoticed, as Zakuulan forces managed to shoot down their shuttle. The pair managed to fight off Eternal Empire troopers before getting the shuttle flying again, arriving in time to rescue Lana and the Outlander from Vaylin. HK-55 personally helped the Outlander up when the hero nearly fell off the boarding ramp.

Unfortunately, the damage Vaylin had done caused the party's shuttle to crash into the Endless Swamp. Surviving the crash, HK-55 announced to his masters that he had detected a large technological object in the swamp. HK-55 accompanied the Outlander in traversing the swamp, while the others stayed behind to conceal evidence of their crash, before they find the object, revealed to be a derelict frigate. When Koth and Lana rejoined them, HK-55 informed him that the architecture of the starship did not resemble Zakuulan design, ancient or modern. Koth identified the ship as Gravestone, the legendary starship that singlehandedly fought the Eternal Fleet and won. The Gravestone's mere existence convinced Koth that their victory against Arcann was assured.

After killing the creatures that had taken residence in the Gravestone, HK-55 took up a patrol routine while his masters gathered parts and supplies, eliminating skytrooper scouts that he encountered. Later returning to the Gravestone to take up a guard post, HK-55 soon spotted an iknayid that somehow evaded his shots. HK-55 furiously chased the creature before running into a whole pack of iknayids, which he promptly dispatched.

No sooner was the Gravestone repaired did the Eternal Empire came bearing down on them. During the fighting, the party was joined by Senya Tirall, a Knight defector. Koth refused to trust as the ex-Knight spent years hunting him down for desertion. HK-55 then reminded his masters that enemy reinforcements were on the way, so Koth tabled the argument as they managed to get the Gravestone off the ground. The Eternal Fleet arrived to stop them and opened fire. HK-55 fought with the Outlander against skytrooper boarding parties as they made their way to the gunner station. Firing the omnicannon, they were able to destroy more than two dozen Eternal Fleet warships before escaping into hyperspace.

Unfortunately, using the omnicannon shorted out the hyperdrive, causing the Gravestone to drop out of hyperspace in the middle of nowhere. While awaiting repairs, HK-55 secretly monitored everyone's activities while exploring the ship. He later ran into the Outlander to ask if the "meatbag bickering" had finally ended. HK-55 then mentioned that several sections of the Gravestone were sealed off before adding that while examining a mysterious monolith to determine if it could be weaponize, he was interrupted by Senya.

After arriving at the shadowport of Asylum and meeting with Koth's crew, Koth requested the Outlander find his engineer Tora. HK-55 accompanied the Outlander because he had Tora's image in his databanks. After fighting through the Starbolts gang, they found Tora in deep trouble with another gang leader named Tanno Vik. When Vik declared his intention to punish Tora for insulting down him and his mother, HK-55 threatened to relieve him of his arms.

After the Outlander resolved the situation, they went to meet with Senya, who wanted to introduce the Outlander to potential allies. She then opened a concealed door but before HK-55 could enter, he was hit with an electromagnetic pulse that incapacitated him and wiped his memory. When HK-55 came to, he began wandering Asylum before Lana found him and brought him back to the Gravestone. Koth quickly assumed that Senya had betrayed them but Lana believed that Senya's contacts were the real danger and asked HK-55 where he last woke up.

Gathering the crew, HK-55 led them to the concealed entrance. Storming the sanctuary, they found the Outlander in a standoff with the Scions of Zakuul. The Scion leader, Heskal, revealed to all that the reason why they couldn't see the Outlander's future is that Emperor Valkorion had survived within the Outlander's mind. HK-55 promptly scanned the Outlander's brain and found no evidence to suggest that another mind could be inhabiting it.

Senya soon delivered another revelation as she identified herself as Arcann and Vaylin's mother. The Outlander's party then returned to the Gravestone, where Koth had it out with Senya for lying to them. HK-55 soon interrupted his tirade, announcing that T7 had passed along a message from the Lady of Sorrow, an elusive information broker operating on Zakuul, who desired to establish a dialogue with the Gravestone's computers. HK-55 then remained behind to purge the remaining alien creatures aboard the ship while the Outlander and Senya traveled to Zakuul to recruit the Lady of Sorrow.

The Lady of Sorrow was later revealed to be an advanced droid named SCORPIO. Upon the Outlander's return to Asylum, SCORPIO requested full access to mainframe, but was delegated to the hyperdrive instead. When HK-55 threatened SCORPIO, she insulted his lack of intelligence.

Soon after the Outlander was called to the Scions' hideout by Heskal Asylum came under attack by Arcann's forces. During the fighting, Arcann's forces took control of the control spar which powered the docking clamps holding the Gravestone, preventing them from escaping. HK-55 fought to meet up with Koth before proceeding to hold off the enemy as the Outlander went ahead to the control spar to free the Gravestone. HK-55 subsequently went on to join his master as the Outlander was confronted by Arcann. After a long, tenuous fight, Arcann was about to unleash a blast of Force energy to kill the Outlander, forcing HK-55 to interpose himself between Arcann and his master. This action saved the Outlander, but caused HK-55 to suffer severe damage and shutdown.

The damage done by Arcann's attack was initially deemed to be irreparable, but Dr. Juvard Illip Oggurobb, chief scientist for the new Alliance that had risen to fight against Zakuul, was able to rebuild HK-55 at his laboratory on Odessen, albeit with some modifications to his chassis. However, HK-55's memory core was not recoverable, resulting in HK's programming being restored to factory settings and complete loss of memory to any events prior to his reactivation. Dr. Oggurobb called on the Outlander to assist in making the final calibrations to HK-55's security and assassination programming. The Outlander initially found HK-55 in the research lab, blasting every droid in the room to scrap while the terrified lab technicians scurried out of his line of fire, having opened fire before his test was ready to secure a tactical advantage. With the Outlander's aid, HK-55 conducted a series of scenarios to refine his targeting subroutines to the Outlander's satisfaction.

At some point, HK-55 woke up in an unknown location, restrained to an interrogation table. He was greeted by an excitable droid named Z0-0M, who released him at his request. Subsequently, a hologram of his captor was revealed to be the terrorist codenamed the Shroud, who revealed that he had copied HK's memory core to give him access to Odessen base, where he was planning on planting bombs to destroy it and the Outlander. The Shroud then signed off, summoning battle droids and mercenaries to subdue HK. Zeeyo chose to defend HK, calling him her friend, and discovered that her chassis contained an impressive array of weapons. Fighting their way through the Shroud's base, the Coil, HK and Zeeyo discovered the Shroud's holorecordings, detailing his unrelenting hatred for the Outlander for ruining his plans of galactic conquest; a museum of droid heads the Shroud had replicated, including a copy of HK-47's head; and a captive Exarch named Malforia. All the while, Zeeyo began experiencing periodic reboots that regularly wiped her memory, which HK theorized was caused by an electrical surge. Additionally, during their confrontation with Malforia, Zeeyo discovers that she's a replicator droid, capable of changing her appearance to resemble anyone she had just met.

Upon ascending from the underwater portion of the Coil to the asteroid in space linked by a skyhook and reaching the hanger, HK and Zeeyo come face-to-face with another HK-55. This HK unit's design was copied from the original HK-55 as he was before the Battle of Asylum, lacking the battlescars and customized repairs the original HK-55 had received since. The duplicate HK-55 declared Zeeyo the Shroud's property, showing them a holorecording of Zeeyo talking to the Shroud, revealing that she was responsible for capturing HK and was planning on wiping his memory with an electrical surge, only to accidentally get affected herself. HK-55 and Zeeyo then fight against his duplicate. However, upon critically damaging the replica HK-55, the base began to initiate its self-destruct and the computer requested the deactivation code. Zeeyo pleaded with the duplicate HK-55 to stop the self-destruct, but he merely replied that the computer wasn't talking to him before expiring. With time running out, HK-55 and Zeeyo defended their escape ship from the mercenaries attempting to take it, before escaping the base's destruction.

Upon returning to Odessen and confronting the Shroud, to the droids' surprise, the Human began to act all nervous, who claimed that Zeeyo was the true Shroud. As he explained, he was the fifty-first person to be surgically altered to resemble her Human male guise, and coached using the very holorecordings the droids discovered at the Coil, as the true Shroud never put herself at risk. No longer wanting to destroy the Alliance base, Zeeyo stepped up to her subordinate and demanded that he stop it. The bomb computer then recognized her as the true Shroud, and deactivated all the bombs. After the imposter was dealt with by HK, he spoke with Z0-0M about her future moments before Lana Beniko, Theron Shan, and the Commander themselves arrived to find two unknown individuals with HK-55 in their War Room. He expressed fatigued and promised to explain the situation.

HK-55 is a companion character in the digital expansion Knights of the Fallen Empire for the video game Star Wars: The Old Republic. He is voiced by Kristoffer Tabori in the game, who also voiced HK-47, HK-50, and HK-51.

HK-55 is the first HK model shown whose primary function is not as an assassin, but as a bodyguard. HK-55 is the second HK unit to refer to organic beings as "meatbags", the first being HK-47. The HK-50s and HK-51s typically simply called them "organics", with the HK-51 referring to its targets as "undesirables" or "troublemakers".

In spite of HK-55's apparent death in Chapter VIII: Taking Flight, players were given a limited offer to regain him as a permanent companion. Players who were subscribed to Star Wars: The Old Republic by January 11, 2016 were given the in-game mission "Arma Rasa", in which they encounter the rebuilt HK-55. Additionally, players who were subscribed to the game from January 11, 2016 to August 1, 2016 received a bonus chapter of story content, labelled "Shroud of Memory". In the chapter, players are given the opportunity to assume the role of HK-55, getting access to unique abilities and dialogue options of the character. Players who were not subscribed to the game at the appropriate dates were forced to purchase it from the Cartel Market.

In Chapter IV: The Gravestone, players speak with HK in the Endless Swamp on Zakuul. During the conversation, players are given an opportunity to reprogram HK to obey their commands, regardless of the impacts they might have on other character. If the reprogramming is done, players can later order HK to kill a group of helpless Zakuulan exiles.

In the mission "Arma Rasa", players reunite with a reconstructed HK-55. Throughout the mission, HK illustrates a number of scenarios with potential targets, in which the players can authorize or forbid HK from eliminating certain people.

In the Bonus Chapter: Shroud of Memory, players take control of HK-55. In the mission, there are generally three different dialogue options of how to portray HK: nice, comical, or mean. When encountering the janitor Jollo, HK can kill or imprison the Twi'lek. Later, during the meeting with Exarch Malforia, HK can kill her immediately, or release her, to which she fights at your side until dying in the vacuum of space. At the end of the Chapter, HK can decide whether to destroy Z0-0M as a threat, or offer her a place in the Alliance.