If you just want to enjoy a movie, you don’t really care who the hero is. If you want to study the structure of a movie as a screenwriter, you need to identify the hero so you can follow the story structure.
In most movies, the hero is the main character with the top billing. However, in the AIDS drama “Philadelphia,” Tom Hanks is the main character, yet he’s not really the hero. The hero is actually Denzel Washington because he’s the one who changes at the end.
Initially, Denzel Washington is homophobic and making fun of gays. He even turns Tom Hanks away because he doesn’t want to work with a gay man, despite being the lawyer on TV willing to take any case just for quick cash. When he sees the discrimination Tom Hanks faces as a gay man, he makes the conscious decision to help and represent him, which forms the basis of Plot Point I that kicks the story into Act II.
The hero is the person who changes as a result of the story. In “Terminator 2,” the hero might appear to be John Connor but it’s actually the good Terminator because he makes the decision to change and learn now to take a human life. In the original “Terminator,” Sarah Connor is the hero because she learns to take control of her life and fight back despite being chased by a robotic killing machine.
In “WALL-E,” it’s hard to say who’s the hero. WALL-E kicks off Act II by making the decision to follow Eve by stowing away on the rocket. However, Eve is the one who changes during the course of the story while WALL-E remains the always likable robot.
The two main characters you need in your story is a hero and a mentor. The hero makes all the decisions that influence the story while the mentor doesn’t change, but helps the hero change.
In a movie like “Star Wars” or “Die Hard,” it’s easy to see who the hero is. However, if your story doesn’t lend itself to an obvious hero, look at which character needs to change and that’s your true hero. Then your secondary major character is most likely the mentor who helps change the hero.