Today is the ‘International Ninja Day’. Ninja 忍者 were covert agents or mercenaries. Most people know them from fiction and movies. Utagawa Kunisada 歌川 国貞 (1786-1865) painted a scene of a ninja on a Kabuki stage about 1830.
If you are interested in ninja and their history you will find a comprehensive article on the English website of wikipedia. See also an article about famous ninjas at https://www.thoughtco.com/famous-ninjas-195587 and about the allegation that maybe everything you know about ninjas is wrong at https://kotaku.com/all-you-know-about-ninja-is-probably-wrong-5932403.