Alright, so Demetri. People sometimes overlook him because he doesn't have a flashy power like Jane or Alec, but in my read of the Volturi, he's basically the linchpin of their enforcement arm. His tracking ability isn't just a party trick; it's a permanent, inescapable threat that undergirds their entire authority. Think about it: if you defy the Volturi and run, there is literally no place on Earth you can hide. That knowledge alone forces compliance before any conflict even starts. It turns their power from just being strong locally to being omnipresent, which is a terrifying psychological lever.
It also dramatically shifts internal dynamics. Aro doesn't just have soldiers; he has a guarantee. This means the more physically powerful members, like Felix, or the psychic weapons, operate with a kind of confidence they wouldn't have otherwise. They can afford to be patient, to let rebellions fester a little, because they know the escape valve is sealed. Demetri turns the Volturi's might from a hammer into a net. It probably even affects how members like Caius or the lesser guards view their own roles—their purpose is to confront, but Demetri's existence means confrontation is always on their terms.
In a weird way, he might also be a point of silent tension. Someone with that absolute, utility-based power has to be fiercely loyal, or he becomes the single biggest threat to the entire structure. I've always wondered if Aro is more personally invested in Demetri's contentment than, say, Jane's. Because if Demetri ever turned, the Volturi's operational foundation crumbles. His influence is quiet, systemic, and utterly indispensable.