Sasaki KojirouWP (佐々木 小次郎WP, Sasaki Kojirō?), Class Name Assassin (アサシン, Asashin?), is the Assassin-class Servant of Caster in the Fifth Holy Grail War of Fate/stay night. He is an irregular Servant that shouldn't exist according to the rules of the Holy Grail War, but Caster's unique nature as a Servant summoning a Servant twists the process.

Assassin is quiet and composed, and while his general tone has a mocking nature to it, he takes it to another level with Caster. He is honorable in battle, always maintaining a degree of bushidoWP, and he is not the type who would attack defenseless women and children. He willingly keeps Saber from exposing her name without any ulterior motives, and he prematurely stops their fight to keep an opponent from spying on her Noble Phantasm and possibly going after the weakened winner of the battle. He has no actual goal in participating in the Holy Grail War. He is not a Heroic Spirit, so there is no advantage to winning for him. He reveals his "true name" without regard for how this might prove disadvantageous for him, and seems disinterested in Caster's promised reward of making him into a real Sasaki. He only guards the gate while eagerly awaiting Servants to battle.