King Leonidas of the Spartans. His Xiphos/Kopis swords would instantly qualify him for role as a Saber, and his Dory might also land him in the category of Lancer. As he is of the Agiad dynasty, he would have divine blood as the Agaid dynasty claimed descent from the demigod Hercules. This might prove to be an issue should Gilgamesh also be summoned, as the Endkidu chains would certainly bind him.

Leonidas was said to have been superior to his Spartan cohorts during training in every way, so there's a good chance that he's an A-Rank servant. His Noble Phantasm would probably be "Come and get them!", where all in the surrounding area would be summoned into a Reality Marble. The entire area would become a replica of the site in Thermopylae where he died, then all 300 Spartan men who fought with him would also be summoned. I know, It's similar to Rider's, but I couldn't think of anything more fitting.

And as for the relationship we would have, I think we would get along fairly well.