It summons these specific types of human-like phantoms that are always one of two variations:
A) A heavy armor dude with an ultra-greatsword
B) Med-armor dude with a large shield and halberd

These enemies also appear in Aldia's Keep in a similar manner as they do during the Mirror Knight boss; through mirrors. They're relatively weak in terms of defense and health, but can throw out some considerable damage.

I managed to beat Mirror Knight on my third try, but I must say he is one of the most difficult bosses in the game, even with NPC aid.