A Shadow Ninja is a type of ninja, that has mastered the element of Shadows. They were thought to be confirmed by Spike Hike in a Tweet when someone asked him about it. He said "Snow First." This resulted in penguins believing that Card-Jitsu Shadow exists.
The power card Mobile Fire Power Card 1 features a Shadow Ninja. This is obtainable through codes in the "Card-Jitsu Water - Second Wave" packs or you may have a chance of getting it by buying Power Cards in the Martial Artworks catalog.
When asked, Sensei said "There might be Shadow some day," giving more evidence that it may be released (and in turn, the items) but not giving the exact date.
There has been a rumor that Tusk might be related to Card-Jitsu Shadow (And in turn, Shadow Ninjas) due to his shadowy spirit that he uses when he escapes at the end of being defeated in Card-Jitsu Snow.
If any piece of the outfit is permed, the player will be banned for 72 hours or forever, as the items are currently marked as bait.