User blog comment:Gundam Legilis/Destiny In Depth/@comment-26989559-20160111143544

I mostly assumed that Emerald was present just to ensure that everything went as planned and to use her illusion magic if absolutely necessary (Since she used it from quite a fair distance in episode 7, it's not too much of a stretch to see it work on Pyrrha from that range) and that Mercury was there as backup just in case something was wrong and was simply the closest person to Ruby at the time. Since she's the leader of the team that's been badgering them all this time, Mercury's probably going to try to kill her, which is why he decides to throw caution to the wind.

Still odd that Emerald is there, though.