They know more about the threat than anyone. Can tell the hero / heroine everything they want to know. They're a cop who's been hunting the Serial Killer for their entire career, or the Great White Hunter who knows more about dangerous animals than anyone. Perhaps they're an expert on ancient curses, or is schooled in the ways of killer robots. Whoever they are, they're the good guy's best and most reliable source of information regarding The Threat, its plans, its motivations, its patterns, its strengths, and its weaknesses. The two important criteria are that this supporting good guy: