Caster of Red

One of the Servants and masterminds of the “Red” camp. He has the lowest position among the masterminds. After all, the way he betrayed his Master after saying “Hmm, this Master seems still and uninteresting. Eh? Saving the entire world? My, that seems fun. I'll help, I'll help!” is essentially the impulsive and unpredictable action of a young man.
His true identity is the famous ultra-literary master, William Shakespeare. There's no need to talk about the details about his background at this point; he's one of the most well-known writers in the world, after all. Writer-type Servants are pretty much all idiosyncratic and hard to deal with, and naturally he's also one of them. He has no fighting power, but he's the best when it comes to powering up his Master. The more of a character that his Master is, the more his writing excels, or so he says. In other words, strengthening his Master and having him fight as an Assassin is Shakespeare's style of fighting in a Holy Grail War. Naturally, no matter how much he strengthens his Master, the chances of them beating a Servant is extremely low—but it's said that he managed to bring about that unlikely possibility just once in one of the subspecies Holy Grail Wars.
He loves the stories he writes beyond anything else, and he spares no love for his characters. On the other hand, he finds mediocrity unpleasant. By the way, his definition of “mediocre” is not based on someone's abilities or appearance. The ones he calls mediocre are the people who don't decide, the people who don't choose, the people who defer and put things on hold, and the people who aren't deeply moved by the miracle of living every day.
Though it's repetitive to say, he has absolutely no fighting power. However, his fame has spread across almost the entire world, and the illusions created by the “theatrical troupe” are a form of magecraft that, though incapable of dealing direct damage, is perfect for wreaking chaos and confusion. Well, he also sometimes sends his own allies into disorder as well, though.
The final words he said at the moment of his death are one of the few true and honest words said by this man who normally glosses everything over with beautiful lines of literary. Shakespeare actively worked as an actor alongside his career as a playwright. So he probably couldn't help but burn with the desire to act as the leading actor on the grand stage of the Great Holy Grail War.
Incidentally, in the prototype version of Fate/Apocrypha, his Noble Phantasm's name was the same, but its role was greatly different. During the initial plot when Shakespeare was supposed to be an ally, there was a scenario where “just as the protagonist is about to die, his Servant Shakespeare activates his Noble Phantasm and somehow manages to find a way to survive,” but the idea was quickly discarded.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia