This... is the NINJA! If the samurai has all honor, then the ninja has none. His methods are silence, secrecy, and stealth. And his weapons — dagger and disguise...
~ Mumm-Ra

Ninjas are common characters in fiction who practices Ninjutsu and can be both the hero and the villain. When they are villains, they are frequently minions of the main villain, often serving as their assassins and enforcers.