Merlin

Merlin is believed to be the son of the Princess of Wales and an Incubus, his mixed blood and his astounding talent for magic make him one of the greatest Mages that ever lived and one of the few qualified to be a Grand Caster.
Like all the most supreme mages he possesses powerful Clairvoyance abilities. Merlin's, however, allows him only to see the present.

He served first Uther Pendragon and then his heir Artoria as a counselor and court mage, aiding her during her reign. He, however, disappeared soon before the fall of Camelot into the mists of Avalon. There, locked in a tower, he is unable to die and observes the world through his clairvoyance.

Since he is not technically dead and registered in the Throne of Heroes, he couldn't be summoned as a Servant, however, the mischievous Merlin has a hobby of bypassing this rule through loopholes.