Much has been written about the protagonist, but few talk about what's called the "influence character." This is a character whose power comes from his or her influence/impact on the protagonist. The influence character has a different worldview than the protagonist, which helps explore the story's theme. This is often who the protagonist is in an important relationship with, in the B story, or perhaps, viewed as a lead role in the B story. It might be a love interest, mentor, friend, sibling, rival, ally, parent, classmate--almost anything. It's someone who has power based on impact. They will challenge the protagonist's perspective, directly or indirectly, either testing the protagonist's resolve or getting them to change. Typically the influence character and protagonist are linked together, usually by a similar goal.
Here are some examples.
In Moana, Moana is the protagonist, and Maui is the influence character.In The Hunger Games, Katniss is the protagonist, and Peeta is the influence character.In The Greatest Showman, P. T. Barnum is the protagonist, and Charity is the influence character.In Hamilton, Hamilton is the protagonist, and Eliza is the influence character.In Songbirds and Snakes, Coriolanus is the protagonist, and Sejanus is the influence character.In Legally Blonde, Elle is the protagonist, and Paulette is the influence character.
This doesn't mean the protagonist isn't influenced by other characters, of course, but these are the (or rather, "primary") influence characters--their relationship with the protagonist influences the outcome of the story in significant ways, and for at least part of the story (if not the whole thing), these two people are usually bound together on a similar course or by a similar end goal. This creates a "we" perspective within the audience. We are trying to do X. We are stuck in the same situation. We need to work together. We need each other.
But this relationship is about more than . . . well . . . just being in a relationship. The protagonist and influence character mirror and foil each other in key ways. Often by the time a writer finishes a professional-level story, he or she will have done this (to some extent), even if he or she isn't aware of it.
Let's talk about the key components of this relationship (concepts courtesy of Dramatica).





