Assassin (Fate/stay night)

Assassin is a Servant in Fate/stay night.

Clothed in a traditional Japanese umanori hakama and kimono with an indigo haori, this Servant bars entry beyond the main gate of the Ryuudouji Temple. Quiet and composed, he maintains a degree of bushido and is eager to spar with other Servants. Although he has the same class, he is not to be confused with another Servant known as True Assassin.

True Identity
Assassin has no identity, though he masquerades as the mythical Japanese swordsman, Sasaki Kojirou. Although his name remains, he is a skilled swordsman whose actual existence is doubtful. A handsome swordsman who carried a 90cm (150cm in Fate/stay night) long blade, Mono-hoshi-zao. He was born sometime during the Eiroku era, and he's said to have became famous around 10~17th year of the Keichou Era, but the truth is unknown. Books that included Kojirou Sasaki's name all contradicted each other in terms of years, and he wasn't even mentioned in his biggest rival, Musashi Miyamoto's book, "Gorinn no sho". According to records, Kojirou was born in Echizen, he studied under the Tomido style, favored a long blade, and after defeating his master's younger brother, he named his style the Ganryu. After that, he traveled around the country and developed his secret sword technique, Swallow Reversal, which could even kill a flying swallow. Later he frightened the warriors all around the country, fought with Musashi Miyamoto on Funajima in order to retain the pride of the lord he worked for, and died. Because of his long blade, he couldn't fight holding his sheath, so he threw it aside during the battle with Musashi, but Musashi saw that and said to the famous "Kojirou, you lose. If you're were planning to win, why did you throw away your sheath?" Kojirou was Musashi's rival, but his figure in Fate/Stay Night is as illusionary as the moon appearing on the water's surface. There was supposedly a person like him but we have to say the story of Kojirou Sasaki known today was made up by people.

In Fate/stay night, the profile of Kojirou in the modern era is an invention, a blend of details from the lives of many nameless and forgotten historical swordsmen. Assassin, a nameless wraith who shared similarities with the legend, assumed the appearance and skills of Kojirou upon his summoning.

Abilities
Saber's first impression of Assassin was that he appears to have no extraordinary ability whatsoever despite wielding a long Japanese sword with a perfect form. She quickly judged the man to have the trait to hide his abilities, something only Servants of Assassin class possess.

Because of that, this Assassin is very untypical. Normally, Servants in Assassin class are the weakest ones in direct confrontation and they defeat enemy Servants through covert actions or by killing their Masters to cut off their prana supply. Kojirou, however, excels in swordsmanship and was able to fight on equal ground with Saber. Though of the Assassin class his swordmanship is on a level beyond even Saber's. Saber also commented that normal attacks won't work against Assassin. In Unlimited Blade Works, he was able to singlehandedly repel several Servants from passing the Ryuudouji temple gates, and even Lancer commented that he wouldn't like fighting him one on one and would prefer to kill him off from distance.

Assassin possessed a sword technique called Tsubame Gaeshi. A Technique that defies the laws of physics, Assassin can simultaneously perform three sword slashes in mutually exclusive directions that cause the effect called Multi-Dimensional Refraction Phenomenon, it reaches the same level of a Noble Phantasm out of pure Godlike Skill instead of Magic. This Technique is design for close range one on one combat and it's said to be unavoidable if there are no errors in its exicution. This technique synchs well with his class of Assassin.

Role
He was summoned by Caster as a guardian and watchman for the front gate of the Ryuudouji Temple. As an improper servant, Assassin is entirely dependent on Caster for mana and cannot move freely beyond the grounds of Ryuudou Temple. Furthermore, he lacks most of the usual parameters and abilities granted to his class. Because of his limited status, Assassin cares little for the Holy Grail, and outside of his duties to Caster, he only seeks to enjoy a decent sword fight. In the Fate scenario, Assassin drives back Saber during her first assault on the Ryuudou Temple. During their second meeting (anime only), he allows Emiya Shirou and Tohsaka Rin to pass in order to engage Saber in a duel in which he is defeated. In Unlimited Blade works he is defeated by Saber on the last night of the Holy Grail War. In the Heaven's Feel scenario, Assassin's flesh serves as Matou Zouken's catalyst for summoning True Assassin.

In Fate/hollow ataraxia, Assassin remains bound to the Ryuudou Temple, and continues to watch over its front portal even though peace has settled over Fuyuki City. He converses with whoever is kind enough to climb up the steps to see him. When Avenger makes his move, Assassin fights the dog-shadows right beside his Master, Caster.