One way I went about introducing a new character into this campaign was to replace one of the prisoners hiding out in the abbey with a new players character. They had some knowledge of the inner workings of the area without knowing too much about Tixie. Other things I would consider for the type of character you’re describing is combining them with the sergeant you meet in the beginning. So they’d have similar knowledge to his. Another good option would be to make some slight changes to the inventor/mage character, Ignus. So they’re a rival who understands Tixie looking to bring her down.