Literal Translations:
* Ninja: This is the most direct, but it's not commonly used in Portuguese. It's mainly used in gaming or anime contexts.
* Ninjutsu: This refers to the art of the ninja, but it's not a direct translation of the word "ninja" itself.
More Descriptive Translations:
* Guerreiro secreto/espião secreto: This translates to "secret warrior/secret spy" and captures the essence of a ninja.
* Assassino/assassina: This translates to "assassin" and is often used to describe a ninja's skills.
* Sombra/fantasma: These words mean "shadow" and "ghost," respectively, and can be used to refer to the ninja's stealth.
Ultimately, the best translation will depend on the specific context and the nuance you want to convey.