That's a great question because it's rarely a 50/50 split, and the imbalance shifts constantly, which is what makes the tension so electric. The 'power' isn't just about physical strength or magic; it's control over information, social standing, and emotional leverage. The villain might hold all the cards in a physical confrontation, but the hero's vulnerability—their connection to a community, their moral code, their loved ones—often becomes a source of power the villain can't comprehend. It's a vulnerability that makes them predictable, yes, but also gives them a resilience the villain, who is often emotionally isolated, lacks.
I think the most compelling moments happen when those roles reverse, even briefly. The hero, cornered and seemingly powerless, makes a choice that reveals a terrifying strength—like sacrificing their own safety to expose the villain's true weakness, which is often their own obsession. Conversely, a glimpse of the villain's vulnerability, maybe a memory of why they became this way, doesn't excuse them but makes the dynamic tragically human. That shared humanity is the ultimate power balance; it's the thread the hero can pull on, and it's the one thing the invincible-seeming villain can't armor against.
A lot of dark romance plays with this by having the 'villain' possess all the societal and physical power, while the 'hero' possesses an emotional fortitude and moral clarity that slowly erodes the villain's control. The power dynamic isn't balanced; it's asymmetrical, and the story is about those scales tipping back and forth until they find a new, unstable equilibrium, often through a fated bond or a forced proximity scenario that exposes the cracks in both of their armors.