Caster of Black is the Caster-class Servant of Roche Frain Yggdmillennia of the Black Faction in the Great Holy Grail War of Fate/Apocrypha.

Caster wears a featureless mask without openings for eyes or a mouth, a full-body suit, and a blue mantle. Due to the mask he wore, it was impossible to tell whether he was afraid or even felt any emotion at all.

Caster is noted to be eccentric, and he sometimes does not speak even when addressed, simply nodding in affirmation. He had a poor constitution and pessimistic bent in life, so he had been reluctant to come in contact with other people. He was able to hold conversation with others, but never placed any emotion into the exchange. Due to his specialty in a single type of thaumaturgy as a magus, he never had worries about the miscellaneous chores of his abode.

(Source: Type Moon Wikia)