(Grand Rapids, MI, USA)
Question: In a story where the main character is the protagonist, does your impact character have to be the antagonist? Answer:
Absolutely not. Traditionally, it has been common in plot-based genre fiction for the impact character to be antagonist, because these stories placed less emphasis on the main character's inner conflict. Combining the protagonist/antagonist relationship with the main character/impact character relationship is a way to simplify things. (Think James Bond vs. Ernst Blofeld in the early bond films, Sherlock Holmes vs. Moriarity, or Batman vs. the Joker.)
However, stories are usually much richer, more emotionally profound when the impact character is someone else and the story of the main character's inner conflict is distinct from (though still connected to) the overall plot. In romances, for example, the impact character is usually the main character's love interest and often these two characters will join forces against the antagonist.
In recent decades, forms of fiction that used to be very simple (for example, comic books) have been putting more emphasis on emotional depth, so it has become common even in these forms for the impact character to not be the antagonist.
For example, in the recent Green Lantern
film, the antagonist was Parallax, a rogue villain who fed on fear. The impact character was Sinestro, a character who represented the principles of willpower and courage. He believed one had to reject fear.
Hal Jordan, the main character, tries to follow the same principles as Sinestro, principles he learned earlier from his father. But Hal knows in
his heart that his fears cannot be suppressed. Eventually, Hal learns to stop fighting this part of himself, accept his fears, and use them to advantage. So he sticks to his original nature.
This forces Sinestro to change, and we see him at the end giving up his old principles and embracing fear as his new principle.
If you consider the 8 classic archetypal characters, the impact character is often the Guardian (the hero's mentor). For example, Obi wan Kenobi is the impact character to Luke Skywalker. But any of the archetypal characters can be the impact character.
Imagine for instance...
A Contagonist who tempts the main character to change his approach.
An Emotion character who shows the main character how to look at the bigger picture.
A Reason character who shows the main character how to focus on the most important task.
A Skeptic who challenges the main character's illusions.
A Sidekick who is smarter than the hero.
Forgive me for going off-topic a little, but even the protagonist can be the impact character in stories where the main character is someone else. For instance, you could tell the story from the point of view of the sidekick whose hero (the protagonist) shows him a better way to live. This has been the case with several of Dr. Who's companions in recent years. Melanie Anne Phillips often cites the novel To Kill a Mockingbird
as a good example of a novel where the protagonist, main character, impact character, and antagonist are all separate characters.