WordNet records no synonym and no antonym for ninja. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a member of the ninja who were trained in martial arts and hired for espionage or sabotage or assassinations; a person skilled in ninjutsu
a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations
Synonyms, antonyms and the broader and narrower relations for ninja come from Princeton University’s WordNet. Full sources and licences.