In many stories, the villain has a goal and the hero is absolutely clueless about the villain. What binds the hero and villain together is when the villain accidentally intrudes into the hero’s life. This often occurs in the middle of Act I (the 15 minute mark) where the villain indirectly sends something into the hero’s life, which the hero will then pursue and protect for the rest of the movie. This item that the villain sends into the hero’s life is the Symbol of Hope.
In “Star Wars,” this Symbol of Hope came from Darth Vader when he forces Princess Leia to send R2D2 away where R2D2 winds up meeting Luke and accidentally displaying the hologram of Princess Leia. From that point on, Princess Leia represents the biggest threat to Darth Vader because she symbolizes the rebellion and has the Death Star plans. Because Princess Leia holds the key to stopping Darth Vader, he’s the biggest threat to her. That’s why Darth Vader constantly tries to defeat her and Luke must constantly save her or the information she represents.
Because the Symbol of Hope is what the hero is chasing and also the most dangerous threat to the villain, this creates the inevitable battle to the death int he end between the hero and villain.
In “WALL-E,” the Symbol of Hope is the plant, which represents the most dangerous threat to the villain. That’s why the villain constantly tries to destroy the plant and the hero keeps trying to save it, which means only one of them can possibly win.
In “The Little Mermaid,” the hero (Ariel the mermaid) saves a prince from drowning, and this prince now becomes her Symbol of Hope that she pursues for the rest of the story. Of course if Ariel should succeed in getting the prince after being turned into a woman, this will foil the villain’s plans (Ursula the sea witch). Therefore both the hero and villain must fight for the Symbol of Hope so only one can win.
The Symbol of Hope comes from the villain indirectly. Sometimes the villain is directly responsible for sending the Symbol of Hope into he hero’s life (“Star Wars”) but sometimes the villain has nothing to do with it but other forces working against the hero send the Symbol of Hope into the hero’s life (“The Little Mermaid”).
Whatever the case, the Symbol of Hope needs to be something the hero and villain are fighting over. For the hero, the Symbol of Hope represents a chance for a better life. For the villain, the Symbol of Hope represents the threat that could stop the villain from attaining a goal. For that reason, both the hero and villain must fight over the Symbol of Hope and in the end, only one can possibly win.