When we were talking about the common theme of father vs. son in stories, I was surprised that no one mentioned the connection between Gandalf and the Balrog. The Balrog consists of two parts: fire and shadow. Gandalf is a "servant of the Secret Fire." This pairing makes for a sort of father/son match, either with Gandalf being the father of the Balrog (making the shadow representative of the here absent mother) or as a son ("servant") of the Balrog.
If one looks at the matter with the opinion that Gandalf represents the father of the Balrog, one could argue that the Balrog is a character foil for Gandalf, who represents fire and good (light). The Balrog represents fire and evil, or shadow (the opposite of good/light). This puts the two and an equal standing on the basis of sharing "fire" and an opposite standing on the basis of good/evil or light/shadow.