Board Thread:Speculation House/@comment-114.79.60.17-20160626164308/@comment-29907776-20180628084420

Maybe his Semblance is knowing what another person's Semblance does? Or something along the lines of the Scan spell in FF8, completely analyzing the strengths and weaknesses of the characters. You see:

1.) Mercury is the only person who figures out what Pyrrha's Semblance is just by experiencing it.

2.) Though Roman was able to block her to an extent in Volume 2, it was Mercury who figured out how to completely counter Ruby's Semblance.

3.) In his fight against Yang (where people have already noted), he left her with enough Aura to be on the defensive once Emerald used her Semblance on her. Had her Aura been any higher she might've not reacted the same way. If her aura was lower, she might've lost instead and Cinder's plan would take a hit.