Mercury Black is an antagonist in Project Yellow. He is an associate of Cinder Fall and partner of Emerald Sustrai.

Mercury is shown to be rather care-free and jokes around when in the company of Emerald, whom 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 drops this and focuses himself on the mission. His carefree attitude however may occasionally slip out during combat During battle, Mercury displays a sense of seriousness and level headedness. He appears to test and gauge his opponent before fully committing to an attack, implying he has a strategic mind. Mercury appears to have no qualms about intimidating others and killing, 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. This is probably due to his father's tutelage as he was most likely taught in the ways of assassinations. He also seems to derive personal enjoyment from the pain of others. Whether this is for his own self-satisfaction or to put his opponents in an unstable state of mind to take advantage of their emotions is unknown.

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