Is ninja a pronoun

No, the Word 'ninja' is a noun; a Word for a medieval Japanese mercenary agent who was trained in the Martial Arts; a Word for a person.

A pronoun is a Word that takes the place of a noun in a sentence.

The pronouns that take the place of the noun 'ninja' are he as a subject and him as an object in a sentence.

Examples:

The model of a ninja at the museum is dressed in full armor. He looks quite fearsome. People would have been intimidated by him.

Note: It is very unlikely that an ancient ninja would be a female. But if there were female ninjas, the pronouns that would take the place of the noun are she as a subject and her as an object in a sentence.