Zarkon is the ruthless and overbearing ruler of the Galra Empire and has been ruling into the present day for 10,000 years.
Originally the Black Paladin of Voltron, Zarkon hungered for power and began to expand his rule. King Alfor of Altea hid the Lions of Voltron from him, and thus Zarkon began his campaign of terror unopposed.
Allura's surprise on learning that Zarkon still lives after so many millennia suggests that his lifespan is not natural to the Galra species; it is currently unknown how he has managed to survive for so long. It is hinted that his absorption and harvesting of Quintessence may be partially responsible due to its regenerative properties, as witnessed by Keith when a container of the substance ruptured and healed his horribly burned hand.
Driven by a "might makes right" sense of entitlement, Zarkon is a firm believer in a survival of the fittest philosophy, espousing that if half a platoon or fleet were slaughtered that the survivors would have been stronger for it. He truly believes that weakness is an infection and must be eradicated and that only the strongest have a right to live and that the weak exist only for the strong to become stronger by stepping along them like a brick road built of their broken bodies.
Zarkon's knowledge and past history as a Black Paladin is repeatedly hinted at, as he repeatedly states he knows the power of Voltron better than anyone. This cryptic phrasing indicates that he has piloted the Black Lion in the past, and given how Zarkon is a calm and collected natural leader, these qualities would certain make him worthy of controlling the head of Voltron. During his battle with Shiro he even expresses a small degree of nostalgia upon seeing the Black Lion. All of these instances point towards a distinctly plausible past as a Paladin. This was explicitly revealed in the last episode of Season 1.
Zarkon is a sadistic and cruel being, conducting gladiatorial arena battles for his amusement. He forces weak and peaceful beings to battle against monstrous fighters, likely due to his hatred for weakness in general. Despite his ostentatious lifestyle and sizable military, he despises waste and inefficiency, opting to fund and sponsor Haggar's experiments in order to harvest Quintessence directly from planets without the cost of mining and colonizing them, so that his resources can be diverted to capturing Voltron instead.
His experiences, due to his unnaturally long lifespan, have left him with a rather calm and confident disposition as there is very little thus far that is seen to fluster or agitate him. Zarkon does, however, detest those who seek greater ambition without the interests of the Galra Empire being in line with those plans. He considers grandstanding to be intolerable and indicative of an inferior leader and military officer.
He also possesses an iron-clad will as nothing has deterred him from conquering the known universe in ten-thousand years. Such patience and fortitude is almost pathological as it indicates an extreme obsession that goes beyond mentally ill.
His obsession with regaining the Black Lion caused him to undergo a campaign that was repeatedly advised against by Haggar, showing how his possessive and one-track mind can cause him to ignore the wisdom of even his most trusted servants and supporters. This lead to several defeats at the hands of the Paladins and the Blade of Marmora agents within his organization. His fight against Voltron in his own Robeast armor is perhaps one of the most significant examples of Zarkon's obsession getting the better of him. Despite it having never been successfully tested, with all test pilots having died, he refused all attempts and suggestions not to use it. His condition after the battle could potentially be attributed as much to this as the damage he took from Voltron's final blow with the blazing sword.
Unlike Shiro, Zarkon believes that the bond with the Black Lion is based entirely upon one's power and will to dominate rather than a mutual trust between Paladin and Lion. This lead to him being defeated time and time again by Shiro in the metaphysical plain where their mental duels to control Black took place, as Zarkon completely ignored Black's nature as a sentient being, therefore Shiro was able to outmatch him with the lion's help.
Zarkon can be described as somewhat psychotic and remorseless, to the point of turning against his own allies if it suits his needs. His unhinged need to absorb more and more quintessence is marked with utter disregard to Haggar, who has been presumably the one to administer these treatments to Zarkon for quite some time, and has been a remarkably loyal servant.
Zarkon is also a genius of unknown measurements. His abilities as a military tactician are beyond reproach, and his millennia-long reign have left him with unsurpassed ability as a leader.
As the former Black Paladin, he possesses the Black Bayard as his personal weapon. His skill with it goes beyond mastery. He is capable of using the Black Bayard to create massive guns, dense energy shields, axes, swords, and chain blades, fluidly changing its form repeatedly and without breaks or hesitation. Given his extremely long life, he has had ample time to gain total control of the weapon and understand all there is to know about wielding it.
Zarkon is able to commune and connect with the Black Lion from distances that are utterly unheard of, and so was able to track it down repeatedly. He is also skilled at using these augmented mental abilities of his to psychically duel someone for control of the lion, which he has shown great proficiency in. However, after losing to Shiro (with the Black Lion's aid) in psychic combat his link to it was greatly diminished even with added power from Haggar and the Druids. Zarkon would only be able to temporarily reconnect with the Lion when Shiro reopened his link to draw the tyrant into a trap.
Zarkon's abilities with the Black Bayard extend to its usage within his giant mech-suit, able to use its standard sword form along with its chain blade mode. This shows not only his great combat skills, but also his capabilities as a pilot as well.
Interestingly, during his battle with Keith, Zarkon displayed the ability to survive in a vacuum. This indicates that Zarkon's Quintessence-enhancement treatments have left him not only with a lifespan that exponentially exceeds that of unaltered Galra biology, but also with the ability to withstand environments that would easily kill any other member of his species.
Zarkon's Quintessence-enhanced body is also capable of withstanding prolonged combat with the Red Lion, indicating a massive enhancement in strength as well, as even Galra ships have been destroyed by less than what Keith unleashed on Zarkon. The limits of the Emperor's augmented biology are unknown, but they are unmatched by any other humanoid in the series thus far.
- This is the first version of Zarkon to pilot a Voltron lion.