Saint Roswald (Rosward Sei) is one of the World Nobles, whom is allowed to reside in the Holy city of Mariejois.
Roswald is noted to be the most responsible of his family seen, as seen when he scolded his children for their behaviour.
He is first seen with his daughter, Saint Shalulia, looking down on his runaway-blown up slave, Dias, who was the Captain of the Acumate Pirates. After his dog urinates on the man, his daughter shoots him with a gun, displaying no remorse afterwards. A demon-like creature is seen behind him. Later in the story, it is revealed that the demon-like creature is in fact, Pirate Captain Jean Bart.
He is then seen at the Human Auctioning House, with his son Saint Charloss and daughter Saint Shalulia. He becomes displeased with his son, Charloss for bidding on Camie.
He is noted for having a "Pirate Captain collection".