Ark, Cas is fine overall but I have a few things you need to touch up a bit. 
I need more on his abilities for example. 
Also, why he uses armour and when (this is an urban fantasy so armour, while not completely out of the question, is peculiar.)
Last but not least - how did he become a teacher. I can help you out with that but Master Melody's need a pretty good reason to let a demon (and a Hellion) teach. You mentioned hunting and that's already ringing alarm bells in my mind.