Board Thread:General Discussion/@comment-14061097-20140709235317/@comment-10390252-20140725090305

My guess is that they're brother and sister in heart. I think they were both street kids that Cinder has raised and trained. As well as programming them to be loyal to her, it also has ensured that they have a near-familial emotional bond.

Mercury clearly likes needling the more emotionally-highly-strung Emerald. However, when the chips are down, they are at each other's side and will back each other up.

Emerald seems the less mature of the two - note her little 'prank' on Roman and the way she acted. Roman may have hit the nail on the head when he called them 'the kids' and made a crack about it being like a divorce. Emerald and Mercury really are reacting to Roman like kids trying to undermine a new stepfather.