A ninja or shinobi was a covert agent, mercenary, or guerrilla warfare expert in feudal Japan. The functions of a ninja included siege and infiltration, ambush, reconnaissance, espionage, deception, and later bodyguarding and their fighting skills in martial arts, including ninjutsu. Their covert methods of waging irregular warfare were deemed dishonorable and beneath the honor of the samurai. Though shinobi proper, as specially trained warriors, spies, and mercenaries, appeared in the 15th century during the Sengoku period, antecedents may have existed as early as the 12th century.