Ninjas were Japanese Spies, Secret Agents, Assassins, Martial Artists & Scientists all in one! They Practice Ninjutsu and Survival Skills. They do amazing science before their own time, They also get bribed by Shoguns to assassinate other Shoguns. They lived in the Medieval Japan but there are ninjas still around but they don't do most of that stuff except practicing Ninjutsu and Survival Skills.
