Castiel mulls that over for a while, considers avoiding the question but if experience with the Winchesters has taught him anything it's that Dean almost never lets things go. ...scratch the 'almost,' he never lets things go. Cas will eventually have to answer and he'll have to do it honestly, since he promised to himself never to lie to Dean again.

"This body is borrowed. When the Leviathan are either killed or returned to Purgatory, I plan to find Jimmy's soul and return this to him."