Mercury is a class-A shapeshifter, able to fully replicate the appearances of others down to voices and mannerisms. The mannerisms thing is mostly a learned habit, but their shapeshifting allows them to become a 100% copy of anyone they've seen, as well as craft new appearances (such as different appearances to use as themselves when walking about normally). Their power also gives them skin that is metallic in appearance. This has little affect on things such as durability, they get hurt the way anyone else does. That being said, when they shapeshift, it appears as if they turn into liquid metal and morph into the new shape, before the color and textures adjust to match the form in mind.

As for limitations, they mostly are only able to turn into other humans/humanoids. There's nothing really stopping them from turning into animals, but they find it extremely difficult. They can, however, adopt animalistic traits such as gills or claws or wings and obtain their intended function. They can turn into forms significantly larger or smaller than they are, but not extremely so. In addition, their internal organs do all function as intended, so while they could keep up an appearance to defy aging, the physical effects of it would still take hold over time. That being said, they can control the placement of certain vital organs, both voluntarily and instinctually, to avoid potential grevious injury.