The Master of Assasin of Red, Shirou was dispatched by the Church as one of the Masters of the Red Faction.

Spoiler

He is actually a Ruler-class Servant of the Einzbern Master‎‎ in the Third Holy Grail War. He was later materialized due to having come into contact with the Greater Grail and managing to receive flesh.

He wants to make use of the Holy Grail War to fulfill his own wish. He has no connection to Shirou Emiya of the Fate/stay night timeline.