Centuries ago (in the Kamakura-Edo period), Ninja served feudal lords or Daimyō as secret agents. Unlike samurai warriors who were bound to fight fairly by a strict code of ethics, Ninja were trained to act as spies, commit sabotage, and carry out assassinations. They had to go through hard training since they were little, and they were eligible to climb on walls, to become stealth mode and hide their existence, to keep balance and walk on a rope, to notice a tiny sound that normal people cannot hear, etc.
