Assassin served as Caster's Servant during the fifth Holy Grail War. He is actually a nameless swordsman who is masquerading as the famous swordsman Kojiro Sasaki. Assassin is soft-spoken, polite, and composed. He faithfully carries out his duty without question and is always on a lookout for a satisfying sword duel.
Assassin is an oddity because his abilities are not typical to that of an Assassin class. He is exceptionally skillful in sword-fighting but the Assassin class is made for stealth. As a skilled swordsman, Assassin has combat instincts and sword striking speed improved by years of intense training and is able to hide any tell of his attack, making him a dangerous opponent in melee combat.
Kojiro Sasaki was born during the Eiroku era (1558 - 1570) and was claimed to had gained fame around the 10th to 17th year of the Keichou era (1605 - 1612). He wields a 90 cm long nodachi named Monohoshizao (translates to Laundry-Hanging Pole).
Kojiro hails from Echizen and studied under the originator of the "Toda" style. He favored the long sword and defeated his master's younger brother. He named his own style "Ganryu" (translates to "Large Rock style"). After that, he traveled the land to test his skills against various swordsmen. He also developed his famous technique, the Tsubame Gaeshi which could kill a swallow in mid-flight.
Kojiro is most famous for his duel against Musashi Miyamoto at Funajima. Because of the length of his sword, Kojiro couldn't hold on to its sheath during battle and he threw it away in front of Musashi. Upon witnessing that, Musashi announced that Kojiro will lose the duel.
Despite his apparent fame, Kojiro's very existence was in doubt because records of his activities contradict each other and he wasn't written about in the book "Gorin No Shou" by his supposed biggest rival Musashi.
Assassin himself was a nameless swordsman who was inspired by the story of Kojiro and decided to practice wielding the nodachi.
Assassin was summoned by Caster to defend the gates to Ryudou Temple, Caster's base of operations. Assassin doesn't really like Caster but obeys her regardless. Caster created a barrier which will rob a Servant of all his or her power if s/he passes through it, and the only spot where the barrier does not cover is the front gate of the temple grounds.
Assassin was first discovered when Saber assaulted Ryudou Temple by herself. She did not sense anything special about Assassin but was still careful about engaging him in combat. Assassin freely introduced himself as Kojiro Sasaki.
Assassin could easily match Saber in pure sword-fighting and she decided to unleash the full power of her sword to fight him. Assassin noticed that a Servant was spying on their duel and requested to end the duel to allow Saber to keep the identity of her sword and herself secret.
He wasn't seen again when Shirou and Saber assaulted the temple during the last day of the war.
Unlimited Blade Works
When Caster took control of Shirou and brought him to the temple, both Saber and Archer went there to rescue him. Assassin allowed Archer through but dueled Saber, preventing her from entering the temple grounds. After Shirou escaped the temple, Saber brought him home but Assassin dueled Archer, preventing from leaving the temple.
On the last day, Shirou, Rin, and Saber assaulted the temple. Assassin was still standing guard there, revealing that although he was Caster's Servant, he was bound to the temple grounds and was fed magical energy that Caster had collected at the temple.
Saber dueled Assassin and managed to defeat him this time due to the imperfection of his Tsubame Gaeshi technique caused by his bent sword resulting from trading blows with Saber.
Early in the story, Assassin was killed by the black shadow and his shell was taken by True Assassin.
Tsubame Gaeshi is a sword technique developed by Kojiro Sasaki. It translates to "Swallow Reversal" which is said to be able to cut a swallow in mid-flight.
Tsubame Gaeshi is an attack that consists of 3 consecutive sword strikes against the opponent that comes from 3 wildly different angles. It gives the opponent no direction to escape and Assassin's phenomenal speed of execution also prevents his opponent from blocking or dodging all 3 strikes.
The Tsubame Gaeshi was developed from the idea of trying to cut a swallow in mid-flight. The swallow could detect and dodge a sword strike by sensing the subtle movement and displacement of the air, thus it is impossible to cut a swallow with a single strike. Assassin then decided that additional strikes are needed but he lacks the speed to execute them. He perfected the sword swinging technique as a human and gained the required speed as a Servant.
Saber pointed out that the sword strikes are actually executed at almost the same time which would be impossible if the technique were bound by the laws of physics. Assassin does not have a Noble Phantasm, but the Tsubame Gaeshi could qualify as one.
A 150 cm long sword named after Kojiro Sasaki's famed 90 cm nodachi. It translates to "Laundry-Hanging Pole", and it was named so due to its excessive length.
The extreme length of the blade makes the sword unwieldy for most people. Assassin is the only swordsman with enough skill and experience to wield it in actual combat, and his ability to stand toe-to-toe against multiple Servants is further testament to his skill with the sword.