The ninja is the assassin of Wandering Ronin. Not meant to be a front-line fighter, the ninja uses stealth to slay his opponents, utilizing special skills and abilities inherent only to that class.

From the Rulebook:

Assassins were once so powerful in the Middle East that Hulagu Khan had to storm their mountain fortress when they demanded tribute. Renaissance Italy and its setting of political intrigue made the courtly killing into an art form.