Kischur Zelretch Schweinorg

The Wizard Marshal, Zelretch of the Jewel.
Also known by the name of Kaleidoscope, he is one of the five magicians.
(He’s also one of the 27 Dead Apostle Ancestors, but since that doesn’t have any bearing on the story of Fate/stay night let’s just gloss over that.)
A magus symbolized by jewels, he is an inscrutable old man that rages at evil and laughs at good.
Though he occasionally takes an apprentice on a whim, he has yet to find a successor.
Long, long ago, he was a monster who fought the king of the vampires in one-on-one combat and pushed back a giant falling rock with sheer strength.
However, he has aged considerably since then, and it seems he can no longer use his magic like he did in his prime.
By the way, whenever he takes an apprentice, they almost always end up being completely ruined.
This turned into something of a dilemma for the department heads at the Association, during the episode Rin related in Sakura’s true ending.
“If we don’t offer up our most promising students, there’s a good chance they won’t come back at all, but if we do offer up our most promising students it’ll be a huge loss if they end up useless!”
Truly, it was an all-or-nothing gamble.

Fate/side material: Fate Encyclopedia