Sidney is the younger identical twin brother of Greg Kasal. He is the Chief of Caduceus USA/Japan. He devoted his life to the study of clinical medicine after the death of his wife. He rarely shows emotion. Like his brother Greg he can stay calm during situations. He appears in Under the Knife, Second Opinion and Under the Knife 2. In Under the Knife 2 his uniform is a different colour. In Under the Knife/Second Opinion he wears a navy suit, with a white dress shirt and a red tie with yellow and black stripes. He has black hair, hazel eyes and wears glasses.