We... know nothing about her, or any of them, really. Urianger is obvious, Y'shtola and Papalymo easy enough to guess, but Thancred's a low-Intelligence high-Wisdom type, if you pardon the D&D terminology. Yda, though... she's not classically smart from what we've seen, but she can stop bullets with her fists, could probably give Godbert a run for his money physically (and that's a lot of money), and nobody but Y'shtola has ever questioned her as an asset to the team.

If I had to guess, her area of expertise could be the underlying science behind, for example, the discipline of the Fists of Rhalgr. In other words, using aether to enhance one's own physical attributes. It may be that she's simply a prodigy, and even though it's the only thing she's good at, she's good enough at it to have earned her mark.