Souichirou Kuzuki (葛木 宗一郎, Kuzuki Sōichirō?) is the acting Master of Caster in the Fifth Holy Grail War of Fate/stay night. He is known as the strict homeroom instructor of Rin's class, who is not a magus, but simply happened upon Caster while she was fading away and agreed to become her Master.

Souichirou is widely regarded as a strict, austere and impersonal man. He is serious, sometimes emotionless and does whatever Caster says in battle even though he does not care about the Holy Grail. He often displays a blank expression.

Souichirou found that when he accomplished his first and only mission, he felt only emptiness rather than any other emotion. Hoping to find meaning in his life, he left the organization and became a school teacher. He found no meaning and found that he could acknowledge only his own existence. He described himself as a "decayed killer" (朽ち果てた殺人鬼, Kuchihateta satsujinki?).[3]

When he met Caster, he felt something move within him. While he had no interest in the coming Holy Grail War, Souichirou helped her just because she asked for it. His motive to fight is similar to Shirou Emiya.[3]