Matou Kariya

The little brother of Matou Shinji's father, Byakuya.
The gutsiest, most rebellious person in the history of the Matou family—against that monstrous old man, just standing up to him and running away was a heroic act of valor. Of course, from Zouken's point of view, no matter how much better Kariya's magic circuits were than his older brother's, they weren't worth the effort it would take to break his will and train him into a proper family head. In fact, at that point he had probably already decided that he had no choice but to bet everything on Byakuya conceiving an heir that would turn everything around. Unfortunately, in the end his longshot gamble backfired spectacularly, and instead of a miracle that would save his lineage, he got Shinji.
By the way, even Kariya's childhood friendship with Aoi was set up by Zouken, who had become aware of the Zenjou family's excellent hereditary trait long before the Tohsaka. However, due to Kariya turning his back on sorcery and getting cockblocked by Tokiomi, the old man's plans once more came to nothing.
…Well, Kariya bringing his beloved Aoi back to some horrid worm pit was never going to happen in the first place. Maybe if the Matou sorcery was a little more normal, he would have been willing to obediently accept his inheritance and become Tokiomi's rival for Aoi's affections.
For a makeshift magus, Kariya's suitability as a Master was exceptional. The fact that he was able to endure the maddened Lancelot's outrageous magical energy consumption for as long as he did is a feat truly deserving of praise. While he might have been better off if Zouken hadn't punished him by forcing him to summon a Berserker, well… I hate to say it, but the only reason Lancelot was able to fight Gilgamesh as well as he did was due to the parameter boost from being maddened. And given the single-minded devotion with which Kariya was pursuing Tokiomi, there's no way you could expect him to have the prudence to avoid that powerhouse team and survive to the very end. When all's said and done, as long as he carried the Matou spirit of putting grudges before everything else, he was probably doomed to failure no matter what.

Fate/Zero material: Fate/Zero Encylopedia