A lot like the
Changelings of Star Trek canon. She is essentially a liquid being, able to increase surface tension strength and simulate bone and muscle. As such, they have no vitals. It is unclear if the first Doppelgängers were naturally occurring or a creation of some magical alteration. They reproduce by merging with one or more other of their race, each imparting part of their mass and memories to the new being.
They have to absorb nutrients, which they can do by eating and drinking in human form or by simply absorbing matter in their 'resting' form which is basically amoebic, and would be done in a body of water, absorbing plant and insect life. That takes quite a bit longer and is rarely done unless they really need to hide. Breathing, however, is mainly for show and to be able to speak.
Of course they are often found as spies and assassins, but they have no special predilection towards these vocations, it's simply what they're best at. Their personalities are as varied as humans, but they find little common ground with most races. They are forever the outsider looking in on races they can mimic but never fully understand.
Left unmolested, a Doppelgänger will live about a century, though due to their nature they often die sooner.
To kill a Doppelgänger isn't hard, if you know how. Dissipate enough of their mass, erode it with acid, or boil it away with enough flame and heat, and they die no harder than most other races. But they are unperturbed by most conventional weapons solids would find intimidating, slashing and piercing weapons are no more than annoyances to them, and all but the largest of greatclubs or mauls has any chance of splashing enough of their mass away from itself to cause it not to be able to pull back together.
Doppelgängers are rare - and of course by their very nature, hard to spot. But to the last they are always curious and always wanting to acquire new knowledge, for themselves and to pass down to their progeny.