Who is The Phantom Ninja?

The Phantom Ninja is a Ninjago book that was released in 2014.It's the tenth volume of the Ninjago graphic novel series.

Description
"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, and Kai — the Masters of Spinjitzu — can save the day . . . or are 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?"