He is the Servant of Flatt Escardos in the Holy Grail War in Snowfield.
He is the embodiment of the mystery behind the legend of Jack the Ripper. Thus he has no true form, and is capable of shapeshifting into any form he desires.
The legend is full of madness, which the Berserker class gives to the Servants in its class. So in an odd twist of fate, this Berserker managed to keep his sanity just because had been summoned as a Berserker and not under another class.