Little is known of Amalgam but it is suggested that she is an alternative version of Rogue as she wears a similar version of her old costume and is able to absorbs the minds and powers of others with a touch. Amalgam apparently travels back and forth and up and down through time, offering "The Choice" to dying mutants. The Choice is to allow her to absorb their powers, personality, essence, etc. and live eternally through her and her fight for mutant-kind.
Major Story Arcs
She has appeared only twice in the X-Men canon, and both times to the X-Man Angel. The first time they met was during the Mutant Massacre, Amalgam had apparently been offering "The Choice" to many of the dying mutants when she stumbled upon Angel, when she attempted to force him to submit, something inside him fought her, and she realized that he wasn't fated to die just yet. She then left and Angel dismissed it as an illusion. She re-appeared to him a few years later when Angel was injured trying to stop a group of terrorist from stealing nuclear weapons but has not appeared again since.
Amalgam is able to aborb the memories and powers from mutants much like Rogue. She can also time travel and change her form to match the appearance of those she has absorbed.