Berserker  

Jack the Ripper, バーサーカー

The character you're referring to is known as "The Jack of All Trades" in the Holy Grail War held in Snowfield. He embodies the enigma surrounding the legend of Jack the Ripper, which makes him capable of transforming into any form he desires. Despite being a Berserker class Servant, who typically experiences madness, this character was able to maintain his sanity due to being summoned as a Berserker rather than under another classification.