Cas did find her demeanor slightly grating, but that was logically because he was both in a rush and hadn't slept since hearing that Mary had gone missing. She was not his own, but she was still Dean's. And despite the somewhat lessened tension between Dean and himself, he still felt as though he needed to prove to his old friend that he was neither going anywhere or going to disappoint him in some unforgivable manner.
"We...rarely work with beings of a supernatural persuasion," he explained. "I understand you have a history with Dean, but Anna and I...we still operate on the level of very few beings outside of each other can be trusted. Now that the option is available to us, it is still strange, but I will admit it was a poor tactical choice. The favor...is simply that you help us locate the suspect in question. And hopefully lead us to where Mary is."
Cas was prepared to offer Phaedra payment in the way of more blood if she required it, but he'd remembered that she'd mentioned feeding infrequently. Also, he suspected she'd find it insulting if he attempted to offer it to her at the moment. In all honesty, he supposed he was likely to have as well.