Ninjas infiltrated enemy territory using ninjutsu.
The main character of that anime is a ninja.
He walked without making a sound, like a ninja.
The children dressed up as ninjas for the party.
He went into the mountains to train as a ninja.
侍 refers to a samurai warrior who openly served a lord, whereas 忍者 refers to a covert agent trained in stealth and espionage.
忍者 are covert agents or spies trained in stealth, distinct from 武士 who were the officially recognized ruling warrior class.
The everyday counter for people: 一人 (ひとり), 二人 (ふたり), 三人 (さんにん).
Used when counting the number of ninja as people, e.g. 忍者一名 (one ninja).
Standard kanji spelling; widely recognized and used in modern contexts.
From 忍 (nin, 'to endure, conceal') + 者 (sha, 'person'). The reading にんじゃ is a regular on'yomi combination.