A Shoulder Devil is a being that is born at the same time as every hero/heroine. They are only visible to the hero/heroine they correspond with and, like the hero/heroine, can come back from the dead. They often will try to lead the heroes/heroines into trouble and will give advice that often leads to mischief. These beings are always dressed in red and have bull horns, otherwise they look exactly like the hero/heroine that they correspond with.
The hero/heroine will often take the Shoulder Devil's advice and get into all sorts of problematic situations. Eventually the hero/heroine will get so fed up with the Shoulder Devil's misleading advice that they will often challenge the Shoulder Devil to mortal combat. Which doesn't mean much in Godville since the heroes'/heroines' gods also have Shoulder Devil's that correspond to them and that resurrect their fallen earthly avatars.