Mike the demon.

Will and Mike lived in different worlds that could be crossed at one line.
Will lived in the heaven of angels, he himself was an angel, his friends, family, etc.
Mike lived in a demon world, and demons hated angels. They were forbidden from entering other demons' territory, otherwise they could be cruelly corrupted or punished.

One day, Will was given a task: to cross the line between worlds (sometimes angels did this) and see what the demons were doing. Will easily slipped into another, hot and red world, when suddenly, the demon Mike noticed him.