Grigori Rasputin

The master of gacha
Alter Ego is an Servant working as one of the three Alter Ego apostles of the Alien God in the Lostbelts. They are working in tangent with the seven Crypters attempting to supplant the Greater History of Man with the Lostbelts. He first appears under the name Kirei Kotomine, acting as an a Executor from the Church tasked with overseeing the investigation into the dissolution of the previous staff of Chaldea for the incoming director Goldolf Musik.

Alter Ego's True Name is Grigori Rasputin,  known as the monster responsible for the Romanov Dynasty's collapse. He is a Pseudo-Servant in the human corpse of Kotomine.

In The Slow Rumble
Grigori was summoned alongside Gilgamesh thanks Nomura using to the mammon machine, but for what purpose the Mad Monk had been brought from the Throne of Heroes?

Trivia

 * Kotomine Kirei's Post-TS design in TFR/Season 6 is based on SMT's Rasputin, because of the Fate/GO connection.