HK-51 is a companion available to all classes for both allegiances. This companion is unlocked by completing a mission series starting with [50H2+] Fatal Errors or [50H2+] The Fatality in Section X. Once the companion has been unlocked, it can be purchased by additional characters in the Character Perks section of the legacy panel for 1,000,000.
"The droid designated HK-51 is cold, calculating and incredibly lethal. Housed in a sleek body resembling an antique Systech Corporation protocol unit, this HK-series assassination droid lay dormant in the hold of an Imperial vessel for many years. Whatever HK-51's original programming dictated, he pledged absolute loyalty to his new master upon his reactivation.
HK-51 is a model of ruthless efficiency. His assassination protocols deteriorate unless they are put to frequent use. This leads to a constant acquisition, pursuit and permanent removal of qualified targets that would be seen as an obsession in any living being. Beyond expressing pride in successful assassinations, HK-51 has little patience for the trivial emotional problems of organics."
Likes: Efficiently eliminating enemies, aggressive pursuit of victory, willingness to set aside bothersome ethics and morality
Dislikes: Being merciful to known enemies, praising other droids or failing to recognize his clear superiority, seeking peaceful solutions over violent ones
[50H2+] Fatal Errors or 
[50H2+] The Fatality in Section X. Once the companion has been unlocked, it can be purchased by additional characters in the Character Perks section of the legacy panel for 
1,000,000.
HK-51 on Jedipedia
