The apocryphal history of Cousin Eliza is that she started life as a drug addicted nobody on the streets of modern day london. She may or may not have been the sacrificial clone of Bernice Summerfield in a bottle universe (going by the name of Christine Summerfield). She may or may not have had a love affair with a Chris Cwej. We catch up with her as Cousin Eliza when she helps defend the Eleven Day Empire from the Sontaran led invasion. From there she travels through eighteenth century London, Egypt and various other parts including Shada to rescue Cousin Justine (and Shuncucker). In order to thwart Sutekh she became possessed by Horus, eventually sacrificing herself to Sutekh to keep him off guard when she kills him earlier in her own personal timeline, after taking care of Lolita (whatever that means). Horus is likely to return - he's done it before. But as to Cousin Eliza - who knows?