Someone who has the ability to mindfuck others. Someone who is stealth enough to complete missions with other ninjas that involve high risk situations. A ninja can usually "ninja" information out of other individuals who are clear 'un-ninjas'.
Ninjas are usually dressed casually, and people don't usually peg these people as actual ninjas, but rather, ordinary and normal girls. Watch out though, there couldbe a ninja at your workplace.
And she could ninja the shit out of you.
co-worker: blah blah blah blah blah
ninja: (in her mind) - good to know I just ninja'd the shit outta you bud. I'm going to go tell my other ninja this classified information.