King of Franks, Father of Europe, Lord of the Twelve Paladins and first Holy Roman Emperor. Charlemagne (or Karl) was perhaps one of the most important rulers to live, being the first emperor in Europe since the fall of of Rome in the 5th century, and having a domain that extended from France's border with Spain to Serbia. Charlemagne and his Peers are the subject of many literary works, the most important of which are the "Songs of Deeds", which were once as popular as the stories of Arthur and the Knights of the Round. A paragon of Christianity who fought against pagans in the name of God.
In truth, a boy who wanted to play with his friends, but was constantly bothered by God's messenger, being told to "go defeat this army", "go conquer that country", "this king needs you aid against those invaders", etc. So, he turned his friends into his greatest knights in order to never be too far-away from them! In between battles, the thirteen of them would gather together, eat, drink (although Charlemagne was not as fond of alcohol as the others), play, and have as much fun as they could.
Charlemagne is much more interested in having fun than in battling for the Holy Grail. In life, he enjoyed eating, reading stories and listening to music (often doing two or even all three at the same time); in the modern era, comic books and manga would particularly capture his interest. A bit lazy, he might end up doing the bare minimum to win the Grail - he is well aware that he is a top-tier Servant, and it is unlikely that a war would have two or more of those, right? Although many parallels can be drawn between him and the King of Knights, Charlemagne did not fail his kingdom, and lived a full life, dying at age 71 with a mighty empire under his command, which would last for nearly 1000 years more. His wish for the Grail is simply to incarnate in the world, and finally get his well-deserved vacation.
Charlemagne can also be summoned under the Lancer, Rider and Ruler classes, although Saber is where he really shines. His corpse is still preserved in the Aachen Cathedral, within a coffin of gold and silver; his throne can also be found there, and as the cathedral was ordered to be built by him, even a fragment of its walls might work as a catalyst.