Theresa Maeve Rourke Cassidy is a fictional comic book superhero appearing in books published by Marvel Comics, usually those belonging to the X-Men family of titles. Debuting under the codename Siryn, she later retired the name and started using Banshee, in honor of her late father, Sean Cassidy, who first used the alias.
Like her father, the X-Men's Banshee, Theresa is an Irish mutant who possesses a "sonic scream" capable of incapacitating and injuring an opponent's hearing and sending powerful vibrations through the air. She can use these vibrations to fly. Her name refers to the Sirens of Greek mythology.
Theresa was raised by Banshee’s cousin and nemesis Black Tom Cassidy without Banshee's knowledge. By her early teens, she left Black Tom and eventually joined the X-Men offshoots X-Force and later X-Factor.