In the fifth Holy Grail War of Fate/stay night, the Servant summoned in the Assassin class is an exceptionally irregular existence. Unlike a typical Holy Grail War where the Assassin is always one of the legendary leaders of the Hashshashin order known as Hassan-i-Sabbah, this Assassin is a different entity entirely. He is the nameless master swordsman who embodies the folk legend of Sasaki Kojirou, the rival of the famous swordsman Miyamoto Musashi. He is summoned not by a proper Master, but by the Servant Caster, Medea, who exploits a loophole in the rules of the war. Using the ancient mountain gate of Ryuudou Temple as a catalyst, she forcibly manifests this wraith-like figure as a "false" Assassin, bound to the very gate that served as his anchor.

This Assassin’s background is shrouded in mystery, as he himself admits he is a forgotten swordsman from a bygone era. The name Sasaki Kojirou is a title granted to him because he is the only existence that fits the legend, a nameless individual who dedicated his life solely to the pursuit of perfecting his swordsmanship. His physical appearance is as striking as his story; he has long, deep-blue hair tied into a high ponytail with a pale purple ribbon, and he wears a sleeveless violet vest over a matching kimono. He is almost always seen with a small twig in his mouth, leaning against the gate of the temple with his arms crossed, a picture of serene composure. His weapon is the washing pole, a nodachi (greatsword) over five feet in length, which he wields with extraordinary grace and speed.

In terms of personality, Assassin is a man of profound contradictions for his class. He is a quiet and introspective individual who loves the simple elegance of nature—flowers, birds, wind, and the moon. Despite his assigned class of Assassin, he despises stealth and ambush. He is, above all, a proud and honorable samurai who prefers a direct, face-to-face duel where skill alone determines the victor. His demeanor is perpetually calm and unruffled, even when grievously wounded or facing a superior opponent. He often speaks in an archaic, slightly teasing manner, famously referring to his summoner, Caster, as a "vixen". This wry humor and unshakable composure make him seem detached, and his outlook on life and death is coldly pragmatic, a product of the violent era he originates from.

Assassin's role in the story is defined by his unique limitation: he cannot leave the immediate vicinity of the temple gate to which he is bound. Consequently, he functions as the unwavering gatekeeper of Ryuudou Temple, which Caster has converted into her fortress. His primary purpose is to intercept and eliminate any enemy Servants or Masters who attempt to storm the temple. He is a defensive bulwark, and he fulfills this duty with absolute dedication, not out of loyalty to Caster, but out of a warrior’s commitment to his given post. His most significant relationship is with his de facto Master, Caster. While he serves her, their bond is one of mutual necessity rather than affection. He frequently mocks her, and she in turn responds with physical punishments, yet they share an understanding as two beings isolated from the normal rules of the Holy Grail War. His other key relationship is with the Saber-class Servant, Artoria Pendragon. He finds in her a kindred spirit, a true swordswoman, and he genuinely delights in their duels, considering her a worthy opponent who allows him to test the very limits of his art.

The most notable development for this Assassin occurs in the Heaven's Feel route of the story. Bound and immobile, he becomes a victim of the war's darker machinations. The true Master of the Assassin class, Zouken Matou, uses his corporeal form as a vessel to summon the genuine Assassin, the Hassan-i-Sabbah known as "True Assassin". This process overwrites and destroys the false Assassin, erasing his existence to make way for a more conventional, and brutal, killer. His role is thus a tragic one; he is a fleeting, phantom swordsman whose only wish was to engage in perfect combat, but who is ultimately disposed of as an irregular pawn.

The true wonder of this character lies in his abilities, which transcend the normal rules of Servant combat. He is a pure swordsman who relies on no magic or legendary weapons, having achieved his feats through endless, fanatical training. His most famous and devastating technique is the secret sword, Tsubame Gaeshi, or Swallow Reversal. This is not a Noble Phantasm in the traditional sense, but a skill so refined that it has reached the realm of true magic. It is a "multi-dimensional refraction phenomenon" that allows him to strike his target from three different directions simultaneously, ignoring the concepts of time and space to deliver three lethal slashes at once. This technique, developed to cut down a flying swallow, is virtually unavoidable, as it attacks from all possible angles of escape. He possesses the skill "Vitrification," a state of perfect mental clarity that renders him immune to all forms of mental interference, including charm and intimidation. He also has "Eye of the Mind (False)" at a high rank, an innate sixth sense that allows him to calmly analyze an opponent's weapon and fighting style after only a few exchanges, making him an incredibly perceptive and dangerous foe. Despite his incredible offensive power, his physical parameters are modest; his endurance is very low, reflecting his nature as a wraith held together by a single purpose, making every battle a high-stakes gamble where a single mistake means death.