She is the wife of the Nathan Spencer. Emily left him somewhere between the time when he became a Bionics user and before he defeated Master-D. They married as soon as they met, but Nathan signed up for the Army and soon left Emily. It was years after Spencer and other Bionics users got betrayed that he learned the truth. Nathan was tricked into service by Super Joe, who had information on Emily's disappearance.
Later, Spencer found out that Emily had been with him the whole time. In order to achieve an operational status, people close to the Bionics user must undergo an unexplained process, where they become an actual part of the Bionic. It is never fully explained what part of Emily is in the arm, and if the rest of her is alive or in what condition.