Board Thread:Dark Souls II/@comment-5264386-20131018105419/@comment-5264386-20131018110427

Mirror Knight Info (15th September, 2013)
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