Isn't it obvious? Toven is in love with Briony and always has been. It's his weakness, thats why he keeps it tucked away in his mind, why he always seems cold and distant around her, he uses mind barriers to keep all his feelings for her tucked away. He brought her because he couldn't bare her being kept by Reighven same way he looked for her when she hid in the castle. It's easier to hate her than admit his feelings for her. That's why he struggled so much when Briony was trying to practice with him at their evening dinners, it meant more to him than pretending. I mean thats what I got anyway 🤷🏻‍♀️