This character makes only a few cammeos in Fate/stay night so here you are a reduced profile concerning the available info.
name: Kishua Zelretch Schweinorg
age: unknown
profession: magician (he practices a particular jewels magic that allows him
to have the access to parallelar worlds and take mana from those dimensions)
personality: Zelretch is an old weird person who seems to be quite positive on
everything. He doesn’t really stand what he doesn’t like and he simply eliminates it
whitouth thinking about it twice. He’s weird and he mainly wants to have fun, in the
ways he prefers (like taking apprentices). He’s like a legendary figure due to his past
experience and he’s somehow worshipped in the restricted circle of magi.
personal story: Zelretch is also known as one of the 27 Ancestors, the person who was able to defeat the king of vampires all by himself. It is said that after that event his powers lowered, although it’s not clear what he can still do right now. As he retired from “pure” action, he decided to took some pupils and try to look for a successor. One of these students was Tohsaka Nagato, one of Rin’s ancestors. In Fate/stay night he’s mentioned in the third route and he also appears in a few flashbacks, when Rin is able to create a copy of his famous sword and he pops out congratulating to her.