A servant that was summoned by Caster for the Holy Grail War. He was ordered to guard the entrance of Ryudoji Temple while Caster
Assassin was summoned by Caster to serve as the guardian of Ryuudouji Temple. He isn't a proper Heroic Spirit since he was summoned by another Servant and as a result, he is limited in his movements to being within vicinity of Ryuudouji Temple and he lacks a number of the skills and parameters that would be expected of his class. His form is based around the legendary Japanese samurai Sasaki Koujirou, a nameless martial artist known in legend for trying to perfect a killing technique on a swallow bird without any records of actual existence.
Assassin stands at 69 inches tall (176 cm) having purple hair and eyes. He is dressed in the attire of a samurai swordsman commonplace from Japan's Eiroku era, the era in which it was believed that the real Sasaki was born.
Assassin is a quiet, composed man who upholds the traditional code of bushido to a degree. Because Assassin is bound to the temple, he has little interest in the Holy Grail and instead seeks to test his skills on those who invade the temple as he enjoys having a good sword fight.
Fate Stay Night covers the Fate route, Unlimited Blade Works movie covers the UBW route, and Heaven's Feel route is still visual novel exclusive.
Assassin had held off Saber during her first attempt to attack the Ryuudouji Temple. He isn't seen in the second attack and was assumed to have vanished after Caster's death. However in the anime adaptation of the route, Assassin appears a second time allowing Rin and Shirou to pass through the temple while he faces Saber, whom he loses against.
In the final events of the Grail War, Assassin appears to fight Saber at the temple when she goes after Gilgamesh. Like their battle in Fate route, Assassin is defeated in this route as well.
As an improper Servant, Assassin has abilities and tactics different from the regular skills and parameters of his class. Rather than being physically weak and relying on covert strategies to dispatch his foes, Assassin excels in swordsmanship and prefers to battle his foes directly. He also cannot draw upon mana from his Master, Caster, to retain his body due to her status as a Servant as well. As a result, his body is retained from the nearby mountain that Ryuudouji Temple is by. As a result, Assassin is still capable of remaining in the living world for as long as he is near the mountain and can even stay alive if Caster is slain. However, the mountain cannot constantly replenish Assassin's mana reserves thus the supply on him is enough to keep his form intact for only 20 days.
The area within the temple and Assassin's swordsmanship serve to make him a formidable defender for Caster. The temple stairs regularly give him a higher position for attack over his foe and the area being covered in Caster's barrier and being a "Servant-killing battlefield" greatly weaken the effects of magic and Noble Phantasms. As a result, Assassin was capable of holding off five of the Servants involved in the Grail War, with only Saber being capable of fatally wounding him with Excalibur due to its high power.
While not technically a Noble Phantasm, Assassin's katana aids him in being the guardian of the Ryuudouji Temple. With it as his weapon, Assassin is capable of delivering strikes with it fast enough where even Saber has difficulty following its movements and relies on technique with it to strike down his foes whenever an opening becomes available. However, the katana will break if it clashes with a western sword like Excalibur due to such swords having more weight and power in their strikes.
Assassin has a secret technique with the sword called Tsubame Gaeshi where he performs three simultaneous sword strikes from all sides on his foe that bend the law of physics as the two later slashes in the technique disregard time and space to close in on their foe to prevent parrying or evasion, making it an ability on par with the power of Noble Phantasms. Because the technique was formed from physical skill instead of magic, it requires no prana for Assassin to use it. However, the technique can only be fully performed on flat ground as the slanted angle of the temple stairs would only allow Assassin to perform two strikes from it.