Assassin: Kensei (Master: Miles Seiler)

Wikipedia:
In ancient Japan, a kensei (剣聖, sword's saint; sometimes rendered in English as Kensai, Ken Sai, Kansei, or Kenshei) was an honorary title given to a warrior of legendary skill in swordsmanship. The literal translation of "kensei" is "sword saint". Thus, the term is considered by some to imply a higher degree of perfection (possibly also encompassing a moral dimension) than the more commonly used kengō (剣豪) or "great sword." Historical kensei are often the founders of popular schools of swordsmanship. Although there is no such written rule, the title carries such prestige that it is commonly understood that there should be no more than one kensei at any given time.


As a Servant:
Kensei is the Assassin summon who has no true identity. His personification is that of the many nameless Samurai of Japan who died with their names and legacy never to be passed on, either in writing or memory. Kensei has no single identity, yet he posesses the skills and qualities common to all honorable samurai, the primary feature being his fine swordsmanship.

His image and powers vary and change with each summoning, but his defining features are long black hair, loose clothing and a long katana for his weapon. He is mainly a single, head-on fighter, the Assassn class only being given to him because of his lack of a true identity. A sturdy fighter more like the Saber class, Kensei in all his incarnations is a reliable servant and a loyal partner.