Very, very slowly, Cassian exhaled through his nose. He needed to keep his head, his wits about him. His father's thoughtful look only meant bad things. There was no way he'd be allowed to stay here, or sent off to fight. His father was stubborn. His fate was inevitable. And honestly, at this point he'd mostly come to terms with it. Though he was bitter that his father was making him play at choosing his own fate. 'Decide what to do with the thief.' He was no idiot, he knew what his father was thinking. All the pawns had been set up in the wrong places, and running away would do no good. And maybe it wouldn't be too bad? Other than the fact that it looked like he would have to travel with a lowly criminal. And a woman, no less. But perhaps she was cunning--though not cunning enough to avoid capture--and if they did come across a Rounen force looking for the 'magic sword', they could wreak havoc among them. That would be entertaining and more worthwhile than just looking for a sword that didn't exist.
He spent a moment in mock thought, mostly trying to organize his words properly and not sound too harsh or too meek. Because when it came down to it, he really didn't want to do this. But how his Father played the words. 'You wish to be king some day.' It felt just the slightest amount humiliating, as if his father was poking fun at him after tying his hands.
"I suppose," He said with a faint grudging to his drawling tone, "While I have little information on the woman, I could take her along in lieu of the knight, if it pleases you, Father." He folded his arms over his chest. "Though I'd like to know her background and speak to her before leaving with her to minimize the chances of her eviscerating me while I sleep." Cassian had little trust in anyone. He'd trusted Serena a small amount, due to her knighthood, and look at what had happened to that. That gave him little reason at all to trust a common thief who may not have any great love for the Kingdom.
If he were to travel with the thief, he'd need to know how to protect himself from her, and how to work with her. It wouldn't do to get on her bad side, because she might not have as many qualms about stabbing him or deserting him on the road and thieving his stuff as a the knight would have.
"Perhaps it would be wise to promise her a sum of money in return for bringing both the sword and my own person back unscathed." He added after a few moments of silent thought. Yes, that's what thieves liked, right? If the reward for not killing him was greater than whatever personal satisfaction it might bring her, then he'd be safer. And it might also stir her to help him fight if they did encounter danger. "How much shall I offer her?" He asked, veiled insecurity in his voice. How much was his life worth to his father? He was the Heir Apparent after all. But, if his father already had the notion of sending him away, perhaps there were other motions in line to make his undoing. Again he wondered if his brother had come back...but he quickly vanquished those thoughts from his mind. Such paranoid ideals would make him weak, he needed to focus on the present.