Theresa Maeve Rourke Cassidy is the mutant daughter of Sean, the mutant who was to become known as the Banshee, and Maeve Rourke Cassidy. Soon after his marriage, Sean Cassidy, an Interpol agent, was sent on an undercover anti-terrorist mission for many months, not knowing when he left that Maeve was pregnant. Shortly after Theresa's birth, Maeve took the infant Theresa with her on a visit to her relatives in Armagh in Northern Ireland. While she was there, Maeve, an innocent bystander, was killed in an IRA (Irish Republican Army) bombing along with a number of other people. No trace of Maeve was found by the authorities and they, and Maeve's relatives who knew of Theresa's existence, assumed that Theresa had also been killed in the explosion and never told Sean that Maeve had had a daughter.