Petra

Twelve-year-old Petra's first and so far only appearance is in the Mystery of the Crystal Castle. Petra and her twin brother Dirk meet Christina and Grant at the castle and help them solve the mystery of the map Grant had found. She and her brother are reveled to have a secret hiding place at the bottom of a dark staircase. They eventually use the map's direction to the top of a turret. There they find the old royal seal and find a chest. Then they open it using a key Grant had found in a secret room in the Singer's Hall, and find the long-lost pearl necklace!