Castor rui Britannia (キャスタール・ルィ・ブリタニア, Kyasutāru ru~i Buritania) is a character exclusive to the Nintendo DS video game and a member of the Britannian Imperial Family. His twin brother is Pollux rui Britannia, he possesses a pair of blue eyes and a much gentler face than his brother. He pilots the Knightmare Frame, Aquila. Castor possesses a Geass ability that allows him to telepathically communicate with his twin brother Pollux.