"Adrian!" One word. And you know the scene. Rocky Balboa, bloodied and broken, has just gone the distance with Apollo Creed, the Heavyweight Champion of the World. It's a split decision. And even though Creed retains his belt, Rocky is the ultimate winner. He goes 15 rounds not just for himself and the people of Philadelphia. He does it for Adrian. And still today when someone yells out her name, you know the film. Rocky is legend not because of the fight. It's time-tested because Adrian was someone worth fighting for.
The lover, a supporting character, is the hero's love interest in the story. Not every hero has a love interest or wants one, nor should every story find a way to force a lover into the plot. However, when a lover fits well into the story, he or she often becomes the hero's security. Without Adrian, clearly Rocky is nothing.
And due to this safe emotional place, it's common for the hero to tell the lover his or her most private and vulnerable thoughts and feelings: sometimes to vent, sometimes to share doubts and fears, and sometimes simply to sit in silence and be understood.
Remember that opposites attract, and putting opposite personalities in an intimate relationship can become a potent recipe for conflict, which is great material for the writer. But a lover's role is not always based from a romantic or sexual nature, and he or she can come in many different forms: as a child (Christopher in The Pursuit of Happyness), a parent (John Hickam in October Sky), even a pet (Skip in My Dog Skip).
And like the other supporting roles, lovers can also create obstacles for the hero by presenting an ultimatum, misunderstanding something, or getting caught by the bad guys.
But most importantly, the lover centers the hero, reminding him/her what is truly important. Remember Jerry MaGuire. After rushing to snag the last flight home during the height of his professional life because he realizes he has no one to share it with, he's in his own living room among a Divorced Women's Group in session looking for his wife. Dorothy enters, and after a lot of words, he simply says, "You complete me." And in the end, that's all that matters.