Board Thread:Dark Souls II/@comment-5264386-20130801122019/@comment-5264386-20130915091602

Just a little tidbit: The devs have stated that the Mirror Knight will be able to summon other players to fight you during the boss fight.

Source
http://www.shacknews.com/article/81109/dark-souls-2-to-enhance-online-play-keeping-isolation-focus