The Rider class Servant of the Fourth Holy Grail War. He was originally supposed to be summoned by Kayneth El-Melloi Archibald, but instead formed a contract with Waver Velvet, who stole the catalyst Kayneth had prepared before the war began.
Going in, the original author entrusted me with just three pieces of information regarding Alexander: his true name, the Noble Phantasm called “Army of the King,” and the fact that there must be a development where Saber comes to reconsider her own kingship through her conflict with him and Gilgamesh.
So, starting from there I figured the most important thing was that he be a totally different sort of king from both Saber and Gilgamesh. That’s why I decided to come up with his general appearance and personality first, and only research his actual history afterward, all the while looking for ways to twist and distort things along the vector I had already imagined for him. But then, the more I read about him, the more I realized Alexander the Great’s actual life was like something out of Shounen Jump. I barely had to change anything about him at all! With the way he faced formidable foes with the utmost respect and behaved more like an adventurer than a statesman, I can’t help but imagine him as a character in one of Hara Tetsuo’s manga. Actually, now that I think about it, It’d be great if Hara Tetsuo made an Alexander the Great themed manga for real! I’m being serious.
His love of the Iliad ran so deep that he once totally disregarded strategy in favor of sightseeing around the city of Troy. He might have been the first celebrity otaku in human history.

Fate/Zero material: Fate/Zero Encylopedia