Куроґірі Сацукі

A sorcerer who pursued eternity and wanted to see the world where everyone did what they wanted to do. He believed that true eternity is one where the observer doesn't change and the object that is observed doesn't change either. He returns people's lost memory and just lets people do what they want to do. Kurogiri is said to be the only mage in the world to be able to speak the ancient "Unified Language". He has several names, "God's Word: Book of Fake god", "Master of Babel: Unified Language Master", "God's Word May Day". As expected from his unusual talent, the Mage's Association branded him for a Sealing Designation, a title 'of greatest honor' bestowed upon a prominent magus.

(Source: Type-Moon Wiki)