River Song (Karla-Gard's universe)
From Tardis Wiki, the free Doctor Who reference
River Song was an alternate version of Melody Pond who hailed from a parallel universe.
She assassinated her universe's version of the Doctor, before turning on her captors during the Massacre of Demon's Run. She acquired a TARDIS and began travelling the universe, living through multiple incarnations including one resembling the River Song of the Doctor's universe, but soon found she was being pursued by the Karla-Gard.
She accidentally slipped through into the Doctor's universe via a rip in dimensions where she encountered an alternate version of herself. Together they defeated the Karla-Gard and she successfully returned to her own universe, where she sought out her own world's Professor Dern. (AUDIO: The Two Rivers)
