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.
Created by writer Chris Claremont and artist Steve Leialoha, Siryn first appeared in Spider-Woman #37 (April 1981), where she appears 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 through 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) through issue #244 (2012), in which she becomes The Morrigan and leaves the team and general publication.
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. Not much later, Maeve dies in an IRA bombing. With no means to contact Sean, his cousin Tom takes care of Theresa. When Sean returns to learn of his wife's death, he is devastated. Before Tom can even tell him of the existence of his daughter, Sean lashes out at Tom with his sonic scream for not having taken better care of Maeve. While Sean flies away in anger, Tom falls into a chasm, breaking his leg as a result of the attack, which leaves him with a limp. Tom swears to make Sean pay and vows to never tell him about his daughter, raising her himself instead.
Tom (now known as Black Tom) trains Theresa as his apprentice in his criminal activities. While still a young teen, Theresa accompanies Black Tom and his then-partner, the Juggernaut, to San Francisco. Here, employing the name Siryn, she aids them in a theft of vibranium, and battles the original Spider-Woman. Due to her inexperience, she performs poorly in battle and is defeated by Spider-Woman again during Black Tom's temporary capture. Initially believing her to be seriously hurt, Black Tom resolves to break all association with her so that she will not be exposed to further danger. While in custody, he exonerates Theresa of responsibility for her crimes, and writes a letter to Sean explaining who she is. The X-Men bring Theresa back to their headquarters, where she is joyfully reunited with her father.