Sasaki is the main Assassin servant summoned during the events in Stay Night. He was summoned under complicated circumstances by the Caster servant, Medea. His sole task is to guard the gate of Ryuudo Temple, where Caster and her Master are stationed. While he does not appear all that much, his main gag in Carnival Phantasm is being unallowed to leave the temple at any point.

I can see Sasaki having anywhere from a Medium Low to Medium High Adult voice.