Ninjas are from Japan. There are females and males ninjas.
Most of the times, ninjas are just used for gathering informations, since they have been trained to shealth their movements.

It was back in 79x-119x A.D. when ninjas were truly born. They put their skills...nonuse..? into real use. However, ninjas do not have any superhuman ability. (And Santa doesn't exist.) While the samurai, a stronger and more popular class in Japan, has some code of honor, ninjas do not follow anything like that.

They do whatever it takes to get their assigned job done: be it attack from behide, poisoning the well, sneaking into someone's house. (Which were against the samurai's code)

The ninjas also don't serve any master. They belong to their clans, and works for money. (The clan leader assigns the jobs) The same ninja might aid you in the morning, and kill you at night, if he's assigned to do that.

That is why most ninjas were feared.