Lord El Melloi II

Waver Velvet, who appeared in Fate/Zero. This is him roughly ten years after those events. He was half-forced to inherit the title of El Melloi II by Reines and spend his days working hard to repay his debt and repair the family's Magic Crest while teaching students as a professor.
To find out more about him, go check out “Character material.” Even in the Fate/Apocrypha world, he fought alongside Rider, Iskander, in a subspecies Holy Grail War against Kayneth—that's the setting here, anyway.
After the end of the novels, Caules Forvedge Yggdmillennia also ends up joining El Melloi II's class.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia