Darius Bertrand is the Shipwright of The Seraph Pirates and one of the two principle shipwrights who designed their ship The Soul Sword. He originally joined the crew out of nessecity having to escape from a vengeful Marine captian who lost badly to him in a game of cards (though Daruis beating him up afterwards when he refused to pay up probably didn't help). So his father gave their ship the Soul Sword to Kai and the crew in exchange for taking his son out to sea to protect him from the vengeful marine captain.