the Main vs. Impact Character trait that inhibits or undermines the effectiveness of that Main vs. Impact Character's Unique Ability
To balance the Main Character's extraordinary status conveyed by his Unique Ability, he must also be shown to be especially vulnerable in one area as well. This vulnerability is called his Critical Flaw. The Main Character's Critical Flaw is his Achilles heel that prevents him from being too one-sided. Just as with Unique Ability, the Critical Flaw can be quite mundane as long as it can threaten him with failure from an unprotected direction. The specific Critical Flaw must be unique to the Main Character in the story. However, the more common the Critical Flaw is to the audience, the more it will identify with the Main Character's predicament. In Start stories, the Critical Flaw inhibits the Main Character from using his Unique Ability. In Stop stories, the Critical Flaw undoes the work done by the Unique Ability after the fact. Only when the Main Character learns to either Start or Stop as required by the story can the Critical Flaw be avoided, allowing his Unique Ability to solve the problem. The Impact Character in any story also has a Unique Ability which makes him uniquely qualified to thwart the Main Character. But in his character as well is a Critical Flaw which prevents him from just totally overwhelming the Main Character. This is again a trait which is unique to this particular character, but its effects are felt in a different area than the Main Character Critical Flaw because of the Impact Character's different purposes.