As a secondary character role, the love interest can serve a variety of functions. At its most basic level, this is simply the individual whom the protagonist cares about. In such stories, the love interest looks attractive yet stays in the background.
There is so much more, however, that you can do with this story role. What if, for example, the antagonist is also in love with the same character? Now we have protagonist and antagonist vying for the affection of this person. Ah, the old love triangle plot structure. Gonna be conflict there!
Or the love interest can bring in a subplot that gives a story additional complexity and texture. For example, in the mystery novel ODDS AGAINST by Dick Francis, protagonist Sid Halley is still in love with his ex-wife Jenny. He knows their breakup is final, yet he is still chained psychologically by the feelings he has for her. At a few key points in the story’s progression, that subplot surfaces, culminating in his meeting Jenny and her new husband. It gives Sid extra dimension as a character plus generates reader sympathy for him.
Winning the love interest’s heart may serve as the reward component of a story’s climax. So that, after the hero has won, some last wrinkle in the relationship is ironed out and now life really will be happily ever after for these characters.
The love interest generally needs to be a character that’s attractive or appealing, worthy of the protagonist’s affection. Otherwise, if the protagonist actively continues to pursue the love interest, readers are going to lose sympathy or even think the protagonist is a dunce.
One of the most tender and sympathetic “doomed” romances is a subplot in THE HUNCHBACK OF NOTRE DAME. The hunchback Quasimodo falls in love with the beautiful gypsy Esmeralda; she cannot love him back. He saves her life; at best, she can only give him a measure of compassion. It’s not her fault. As readers of Victor Hugo or as an audience watching the movie version, we don’t necessarily want Quasimodo and Esmeralda to fall in love with each other. She is already married and loves her husband. Yet through meeting Esmeralda, Quasimodo grows as a character. We see his humanity despite his hideous face … just as Esmeralda comes to recognize it.