Ninja Kirby is one of Kirby's many copy abilities, first introduced in Kirby Super Star. It is available from the enemies Biospark and Moonja as well as Spinni's projectiles.
Super Star / Squeak Squad
Return to Dream Land
Super Smash Bros. series
In Super Smash Bros. for Wii U in appears as a trophy
"Making his debut in Kirby's Return to Dream Land, the Moonja enemy looks about as ninja as you can get. If you copy its ability, Kirby can become a master of the ninja arts! With a variety of useful skills like Knife Throw and Blossom Storm, you might want to remain a ninja forever!"