Emiya Kiritsugu

A freelance magus, who died at the age of 34.
For better or worse, the person that decided Shirou’s way of life.
He was Emiya Shirou’s adoptive father, and the Master of Saber in the previous Holy Grail War, which he fought his way through to the very end.
He favored guns, and used a peculiar magecraft called Innate Time Control.
His aptitude as a Master was normal, but he was the best of the best when it came to “killing hostile magi.”
Basically, he was the Association’s hitman, and it seems the Einzberns sought him out to make use of those skills.
It seems that over the course of the Holy Grail War, he tricked an enemy Master, took his lover hostage, blew up a building, and was generally nasty enough that you could almost mistake him for a young Kotomine.
…Well, Kotomine himself was young back then, too.
Perhaps it’s the fate of lady’s man, but he casually kept both a wife and lover.
It seems that when he wasn’t working as a magus, he was extremely chivalrous. He used to tell the young Shirou to “never make a woman cry, since later there will be hell to pay.”
…Huh? Did this guy actually like women at all?
At any rate, after the Holy Grail War he was stained by “All the World’s Evil” called forth by Kotomine, which ultimately caused his death.

Fate/side material: Fate Encyclopedia

The protagonist of Fate/Zero’s prologue and epilogue, more or less.
He is the epitome of pragmatism, willing to do anything to achieve his goals. He is an idealist that dreams of world peace, but his method of attaining it is that of a cold and ruthless realist. He loves humanity, but he would not hesitate for a second to kill any single person. But though he lacks both hesitation and doubt, he cannot throw away his sadness.
Essentially, he’s a magical 007, who treats sorcery as a mere tool and has no qualms using modern technology when it’s more effective to do so. I really wanted to have him go on a rampage in a fully-loaded spy car, but I guess if you think about it Saber’s V-Max sort of filled that role.
Though a young boy once looked up to him as a “superhero,” he himself lost that admiration at an early stage, eventually coming to curse the very concept of justice itself. The only thing that kept him going was a desire that the sacrifices he had already made not be in
vain. Thanks to this pessimistic attitude, his rank as a “superhero” is far inferior to the Heroic Spirit that shares his last name. Perhaps if he had a little more of the evil charisma of a last boss he would have been successful, but at the end of the day what he really wanted was just to pass away the days with his family in tranquil serenity.
Just like an eroge protagonist, he emitted a mysterious radiation that attracted women to him, but sadly everyone ensnared by it inevitably met a terrible end. The only woman capable of smashing through this curse was the legendary Tiger of Fuyuki. Wow, Fuji-nee, you’re so amazing!
If you count backwards from when he died, it turns out that his most active time on the battlefield was actually in his late teens. But hey, that’s not so strange, you know. Just look at Yagashira Dai or Sagara Sousuke. There are lots of heroes like that in this world.

Fate/Zero material: Fate/Zero Encylopedia