For Whom the Bell Tolls
We can't really justify calling Maria a protagonist, because she's essentially a moon orbiting the planet of Robert Jordan. But she's pretty clearly The Love Interest, and her love for Robert Jordan can be looked at as his second great mentor. It's what really brings about his transformation in those few days up in the mountains. If you read the war as an antagonist, Maria's also its most obvious victim, both because of the events in her past and her loss of Robert Jordan.