I've always been fascinated by how authors try to make these characters feel plausible. The most convincing legend prodigies aren't just powerful from page one; they're defined by an obsessive, almost terrifying focus on a single craft or magic. It's not just talent, it's a monomania that isolates them. Think Kvothe from 'The Name of the Wind'—his genius is tied to his relentless, sometimes arrogant, pursuit of knowledge and music, which constantly gets him into trouble. The trait I find most defining is that their prodigy status warps their relationships; everyone either wants to use them, worship them, or break them.
That warping is what separates a Mary Sue from a compelling legend. Their 'gift' often comes with a massive flaw that's the dark side of the same coin—arrogance, social blindness, a hidden fragility. The legend part kicks in when their specific, narrow genius is the only thing that can solve a problem nobody else even understands. It's less about being good at everything and more about being peerlessly, dangerously good at one weird thing that the world suddenly needs.