Assassin (アサシン, Asashin?, Asashin), designated as True Assassin (真アサシン, Shin Asashin?) in reference materials, is the Assassin-class Servant of Zouken Matou in the Heaven's Feel route of Fate/stay night's Fifth Holy Grail War. He is designated as True Assassin because he is the proper Heroic Spirit meant to be summoned under the Assassin class rather than the aberration in the War, Sasaki Kojiro, summoned as an irregular Assassin by Caster. He is also a Masterless Servant in the Subcategory Holy Grail War of Fate/Labyrinth, and one of the Servants able to be summoned by the Protagonist of the Grand Order conflicts of Fate/Grand Order.