Mercury Black/Affiliation

Cinder Fall
Mercury was recruited by Cinder after the death of his father, Marcus Black. Cinder appeared to find a use for his skills as an assassin and offered him a place in her group.

He appears to be quite loyal to Cinder, following her orders dutifully, but is not as outwardly devoted to Cinder's cause as Emerald, being more laid back and casual.

Emerald Sustrai
Emerald appears to be Mercury's partner. The two seem have an easygoing relationship, though Mercury is shown to actively try to annoy her, more often than not succeeding.

This leads Emerald to be willing to bribe Mercury with stolen money she acquired; however, when it matters they work well together, even joking with each other after having killed Tukson earlier.

Roman Torchwick
Mercury and Roman don't see eye to eye and Roman even goes as far as to brandish his cane in a threatening manner towards Mercury who mocked him moments earlier.

Mercury is shown to constantly mock Roman at any given chance, especially after he and Emerald killed Tukson, bringing up that Roman was supposed to take care of it.

Neopolitan
They are participating together with Emerald and Cinder in the Vytal Festival Tournament.

Ruby Rose
Not much interaction has happened between the two of them but Mercury was the one who kept Ruby from saving Penny Polendina during the Vytal Festival Tournament. Mercury was delighted by Ruby's sadness over Penny's death.

Marcus Black
Marcus was Mercury's father. Not much about their relationship is known, but it is likely Marcus, a skilled assassin, trained Mercury in fighting. Ultimately, Mercury killed his father, for reasons unknown. However, it is presumed from the song I'm the One that Marcus abused and mistreated Mercury so that in the end, it's made very clear that Mercury despised him, spitting disrespectfully shortly after killing him.

Qrow Branwen
Mercury tells Cinder that he saw Qrow at Beacon Academy. He knew it was Qrow by his hair, smell, scythe and previous encounter.

[Pyrrha Nikos]]
During the episode Extracurricular Mercury tests Pyrrha's skill in combat, while not revealing too much about his own fighting style. He then concludes that Cinder Fall should be able to take Pyrhha Nikos no problem, as Cinder is seen defeating Pyrrha in End of the Beginning