Theresa Maeve Rourke Cassidy is a superhero appearing in American comic books published by Marvel Comics. The character is depicted most commonly in association with the X-Men. Created by writer Chris Claremont and artist Steve Leialoha, the character first appeared in Spider-Woman #37 (April 1981). She belongs to a subspecies of humans called mutants, who are born with superhuman abilities. Debuting under the codename Siryn, she later retired the name and went by Banshee, in honor of her late father, Sean Cassidy, who first used the alias. Theresa possesses a sonic voice providing various effects, with her name referring to the Sirens of Greek mythology and their hypnotic voices. Publication history Siryn was created by writer Chris Claremont and artist Steve Leialoha, and first appeared in Spider-Woman #37 (April 1981) as a villain. Along with many young mutants at the time such as Sunspot and Multiple Man, she featured in the limited series Fallen Angels (1987). She was one of the first members of X-Force (vol. 1) (1991), beginning in issue #3, and remained a regular in that book until issue #100 (2000). As a love interest of Deadpool, she concurrently appeared in one of his earliest solo titles—Deadpool: Sins of the Past (1994), and sporadically appeared in Deadpool (vol. 1) (1997) and Cable and Deadpool (vol. 1) (2005). In 2006, she rejoined forces with Multiple Man as a member of X-Factor (vol. 3) up until issue #244 (2012), in which she becomes The Morrigan and leaves the team and general publication. In 2021, Siryn appeared in an arc of a new volume of X-Factor (vol. 4, #6-9), which followed up on and resolved her connection to The Morrigan. Fictional character biography Early life While Sean Cassidy was working on a deep undercover mission for Interpol, his wife Maeve gave birth to their daughter, Theresa, at Cassidy Keep, County Mayo, Ireland. Maeve later dies in an IRA bombing, and with no means to contact Sean, his cousin Tom takes care of Theresa. When Sean returns and learns of his wife's death, he is devastated. Before Tom can tell him about Theresa, Sean lashes out at Tom with his sonic scream for not having taken better care of Maeve. As Sean flies away in anger, Tom falls into a chasm and breaks his leg, leaving him with a limp. Tom swears to make Sean pay and vows to never tell him