Who is the Phantom Ninja?

A factory in Ninjago Village is under siege by an unstoppable new menace, with the beloved mayor and his daughter held hostage! Only Nya, Jay, Cole, Zane, and Kai — the Masters of Spinjitzu — can save the day . . . or can they? Much to the brave warriors’ astonishment, a new gray ninja is on the scene! Is the Phantom Ninja friend or foe, and will his reckless fighting style sabotage the rescue mission before it even starts?