In Mercury's first appearance in "Black and White", he is shown to be a pale, well-built young man with gray eyes and, while draped in shadow, somehow purple hair (revealed to actually be gray in the credits of the same episode), partially slicked back while unkempt at the front. He also wears a slate gray and black two-tone partial-zip jacket that covers his upper body. He is stated to be in the age range of a Huntsman Academy student.
Mercury's appearance does not change much leading into Volume 2, though his hair and eyes become a silvery-gray. He also wears a single notched belt with what appears to be a sash or bandana draped over it. Protecting his arms are a set of rerebraces and vambraces.
He has prosthetic legs that attach part-way up his thighs. These legs are usually hidden from view due to him wearing pants and a pair of shoes. His armor hides what appears to be oddly-shaped scars on the outer sides and backs of both of his arms.
The World of RWBY: The Official Companion describes Mercury as a "skinny guy."
In "Beginning of the End", he was first seen wearing his signature zip jacket with the collar zipped all the way up, minus his armor. His pants were dark orange and he wore a pair of black boots.
In "Best Day Ever", Mercury is shown to be rather care-free and jokes around when in the company of Emerald, who he makes an effort to annoy. He seems to opt for excitement and action; he dislikes books without pictures and tells Emerald he finds the city boring. However, when it comes to matters at hand, Mercury displays a sense of seriousness and levelheadedness. However, his carefree attitude may occasionally slip out during combat, such as when he taunted Ruby Rose during both their encounters once he got the advantage in their skirmish. Based on his fight with Pyrrha Nikos, he appears to test and gauge his opponent before fully committing to an attack, showing he has a strategic mind and cunning.
Mercury also seems to derive personal enjoyment from the pain of others, showing no qualms about intimidating and killing others, even light-heartedly joking about it afterward with Emerald. Although subtle, he also speaks in a derogatory fashion about Tukson even after having murdered him in cold blood. He, however, despite his sadistic streak, he is not incapable of caring for others. He takes on a soothing tone when convincing Emerald to retreat from battle while she grieves over losing Cinder.
Despite his usual carefree demeanor, in "The Next Step", it is shown that Mercury does not take insults or disrespect well, responding to Arthur Watts' insults with an eagerness to attack him until Emerald stops him. He is usually outspoken, but when he is with Salem's Inner Circle, he refrains from speaking. This was shown again in "Lost" when he lashed out at and attacked Tyrian Callows when the latter mocked him for being a child that feels like a "big bad man" and correctly assuming all Mercury knows in life are pain and violence, condescendingly calling his life 'such a tragedy'. Unlike Emerald, Mercury also realizes that Cinder never truly cared for him and that in the end, their relationship was purely business, possibly because he either recognized her subtle manipulations as another form of abuse or he simply always knew he was just a pawn to her. Despite this, he is very compliant with her orders.
Mercury carries an immense bitterness within him due to his abusive upbringing by his assassin father, Marcus Black. This bitterness holds over due to the fact that Marcus used his Semblance to steal Mercury's, which he said would return if Mercury became strong enough, which he proved the moment he killed his father. This turned out to be a lie, and Mercury feels as though he has had to work harder than any other member of Salem's Inner Circle due to his stolen Semblance. His father abused him physically and emotionally and that he used to constantly beat him up and call it 'training' while telling him that he was weak and relying on crutches, making it obvious he suffered a miserable upbringing filled with hate and misery. In the same episode, Tyrian later alludes that the only reason Mercury is on their side is because of Marcus' influence and that he is now too afraid to leave the few things he knows: pain and violence.
Powers and Abilities
As seen in his fight with Pyrrha in "Extracurricular", Mercury is shown to be a very acrobatic and agile fighter who relies upon quickly overpowering his opponent with complex kicks that imply a near-mastery of martial arts that resemble Tae-Kwon-Do. He is also capable of a break-dancing style, similar to Capoeira and makes use of Muay Thai as well as shown in the same fight. Despite his focus on kicks, Mercury also uses his hands to grapple, block or redirect enemy attacks.
His experience and skill are aggressive and effective enough to drive even fighters of a high caliber, like Pyrrha, into a defensive position. He possesses great speed which allowed him to outrun and evade the rapid-fire bullets from Gianduja even at point-blank range. His strength is seen when he makes Yatsuhachi stagger on his feet and later knocks him out with ease despite the other's towering size and heavy build. Due to his fighting style, Mercury relies on his swift and quick reflexes to counter and block his opponent's attacks as first demonstrated during his fight with Yang, altering the course of her punches and using his footwork to raise her fists and kick underneath or around her attacks.
Mercury is also known to be observant, tactical and analytical; he notices and deduces the capabilities and limits of Pyrrha's Polarity Semblance after she uses it once during their brief fight. He demonstrates his observational skills once more when he tracks down and counters Ruby's movements when using her Semblance, an ability so fast none of Team JNPR could stop during their brief fight against Team RWBY in "Best Day Ever".
- Main article: Talaria
Mercury's weapon of choice is a set of boots named Talaria that are equipped with a firing mechanism built into them similar to Ember Celica.
Mercury has a pair of robotic legs, which he acquired at some currently unknown point in time. His prosthetics make up most of his leg, including a portion of his thigh and everything below the knee. His prosthetics are also quite durable, being able to withstand significant attacks such as the powers of a Maiden.
Among Mercury's skills, his talents at information gathering and subterfuge would appear to stem from his training as an assassin. While undercover at Beacon Academy, he strategically spars with Pyrrha Nikos in order to gather information on students' Semblances to aid in the initiation of the Fall of Beacon, being able to put on the convincing facade of a well-meaning - if somewhat cocky - student of Haven Academy.
In the episode "Lost", Mercury reveals that his father stole his Semblance when he unlocked it. His father used his own Semblance to steal Mercury's and claimed that he would give it back when Mercury became stronger. However, Mercury never got his Semblance back despite getting stronger and after killing his father, Mercury still has no chance of regaining it. Not much is known about Mercury's Semblance otherwise.
- Mercury alludes to Mercury from Roman mythology. For more information on this allusion, as well as other choices the creators made for this character, see Mercury Black/Behind the Scenes.
- Gray and Miles described Mercury to J.J. as "a hipster douchebag, but he's cool."
- In an interview with AfterBuzz TV, J.J. claims that if Mercury were going all-out, he would have been able to defeat Pyrrha, though he admits this claim is biased. When describing his voice acting for the character, he says that it is basically him doing a bad impression of Jack Nicholson.