A ninja is a kind specialist, he only has a defined purpose.
A ninja is silent, effective and has always his target in head.
A ninja disappears if the work is done, and you won‘t know that he was ever here after he has done his work.
You have to give ninjas clear instructions, and the right weapons.
If you give a ninja bad weapons, he will throw something to you when he is using those bad weapons.
He will report fast, if something is wrong.