Assassin is a fictitious character from the Japanese visual novel and anime series Fate/stay night by TYPE-MOON.

Clothed in a traditional Japanese umanori hakama and kimono with an indigo haori, this handsome Servant bars entry beyond the main gate of the Ryūdōji 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.

The Seiyū for Assassin is Shinichiro Miki. The Voice actor for Assassin is David Vincent.

Source: Wikipedia