Cas is treated as a secondary character who doesn't appear at all in fics set in the first three seasons of the show and is also absent the majority of the time thereafter. And is sometimes an antagonist or not aligned with Sam and Dean.

Oh, you weren't asking about canon?

I hope, if you write fic, that you never fail to feature Bobby, and that you remember that he is as important a presence in Dean's life as Castiel.