The below description is taken from LEGO.com.
"Like a shadow in the night, I - oops!"
Stealthy. Sneaky. A cunning master of martial arts. None of these things describes the Ninja, although he sure wishes they did. He tries his best to dramatically blend into the shadows or hide in plain sight like all the other ninjas can, but no matter what he does, he always seems to mess things up. The truth is that he's just plain clumsy, and no matter how gracefully he may start a back-flip, creep along the rafters or launch himself into an epic sword battle, he always manages to trip over his own feet, fall off the ceiling, or bonk himself on the head with his ancient and mystical blade. Also, he's afraid of the dark.
No one is quite sure how the Ninja ever managed to graduate from ninja school. But no matter how bad he may be at his chosen profession, he always keeps trying and never gives up, convinced after each blunder that the next time, he will prove himself a true ninja warrior for the ages.