"ninja" is a character costume in StrikeForce Kitty.
The fox wearing the costume is found on level three with "47" right after your cats start the level. Wearing the set requires defeating the fox multiple times until all three pieces are obtained, or by chests.
Costume set contains three pieces: mask, robe, and katana. "ninja" does melee damage and has the special ability to deal double damage at a 40% chance.