Formal/Literal:
* 忍者 (rěn zhě): This is the most direct translation, using the Japanese word "忍者" and its pronunciation. It's often used in martial arts contexts or when referring to ninjas specifically.
* 暗杀者 (àn shā zhě): This translates to "assassin" and might be suitable if you're focusing on the stealthy and deadly aspects of a ninja.
* 特工 (tè gōng): This means "secret agent" and could be used if you want to emphasize the espionage and intelligence gathering roles of a ninja.
Informal/More Figurative:
* 神秘高手 (mì shén gāo shǒu): This means "mysterious expert" and could be used to describe a ninja's skills and abilities.
* 武林高手 (wǔ lín gāo shǒu): This translates to "martial arts expert" and might be suitable if you're talking about a ninja's fighting skills.
The best translation will depend on the specific context and what aspects of the ninja you want to emphasize.