Nagato Toosaka

Nagato was the head of the Tohsaka family around the 1790s. Though the Tohsaka family of the time were traditionally hidden Christians practicing despite the ban on Christianity, Nagato and his clan came upon the world of sorcery after "strange old man appeared out of nowhere and led them astray.". Nagato first encountered Kischur Zelretch Schweinorg during a journey, and for having picked up the hotel bill, Zelretch taught Nagato something of magecraft. Though Nagato was officially viewed as a pupil, it was more of Nagato being a "follower whom he looked out for every now and then" in Zelretch's eyes.

(Source: Type-Moon Wiki)