A ninja (忍者) or shinobi (忍び) was a covert agent or mercenary in feudal Japan. The functions of the ninja included espionage, sabotage, infiltration, assassination and guerrilla warfare.[1] Their covert methods of waging irregular warfare were deemed dishonorable and beneath the samurai-caste, who observed strict rules about honor and combat.[2] The shinobi proper, a specially trained group of spies and mercenaries, appeared in the 15th century during the Sengoku period,[3] but antecedents may have existed in the 14th century,[4] and possibly in the 12th century (Heian or early Kamakura era).