Note: The current story-arc begins at Strip 621
First off, I'd like everyone to see the Casey and Andy Cast Song Video made by forum goer JVM:
It could be argued that the Cauldron of Fate is a character. It talks, after all, and has a personality. Also, it was in several strips before the cast song was ever made. So it's one of the few "characters" accidentally omitted from the Cast Song.