Rebecca was Bernice Summerfield's childhood doll.
She was with Claire Summerfield, Benny's mother, when Claire was exterminated by the Daleks. The painful, passionate memory of Rebecca, embodying that of mother's death, helped jolt Bernice out of Psyche Dalek Three's control on Spiridon once the Seventh Doctor had jammed its transmitters, returning her free will and awareness to Benny. (COMIC: Emperor of the Daleks!)
In one possible timeline, Benny would later name her daughter with Jason Kane, Rebecca Kane-Summerfield. (PROSE: The Shape of the Hole)