Gustacio is an elder mage in Moonshade on Serpent Isle. In his old age he requires the use of a cane to walk and, unlike many other mages in Moonshade, is incorrupt and of an honest disposition. After arriving in Moonshade in Ultima VII Part Two, the Avatar attempted to speak with Gustacio. However, the mage was too busy for conversation, being preoccupied with the mystery of the missing Edrin. During the banquet in the MageLord Filbercio's palace, Gustacio actively protected the Avatar from Rotoluncia's attacks. However, he later knew it useless to confront the MageLord when the Avatar was sentenced to rot in the Mountains of Freedom.