Real name: Theresa Rourke, a.k.a. Theresa Rourke Cassidy
Other aliases: None known
Identity: Secret
Occupation: (current) Adventurer, (former) Professional criminal
Legal status: Citizen of Ireland with no known criminal record, still a minor
Place of birth: Cassidy Keep, County Mayo, Ireland
Marital status: Single
Known relatives: Sean Cassidy (Banshee, father), Maeve Rourke Cassidy (mother, deceased), Black Tom Cassidy (cousin)
Base of operations: (current) The Xavier Institute, Salem Center, Westchester County, New York State, (former) Cassidy Keep, County Mayo, Ireland; The Beat Street Club, New York City; Muir Island, Scotland;
Group affiliation: (current) X-Force, (former) Ally of Black Tom and the Juggernaut, Fallen Angels
First appearance: SPIDER-WOMAN (first series) #37

History: Theresa Rourke is the mutant daughter of Sean Cassidy, the mutant now known as the Banshee, and Maeve Rourke. At the time of his marriage, Sean Cassidy worked as an Interpol agent, and not long after was sent on an undercover anti-terrorist mission which kept him out of touch with his family for about a year and a half. Neither Sean nor Maeve knew at the time he left that she was pregnant. Maeve remained at Cassidy Keep, the family castle, during her pregnancy and the time immediately following Theresa's birth and so relatively few people knew of Theresa's existence. Sometime later Maeve was killed in an IRA (Irish Republican Army) bombing along with a number of other people. Those who knew of Theresa's existence assumed that she too had been killed in the explosion, which had left the corpses of its many victims unrecognizable, and decided not to tell Sean Cassidy that Maeve had had a daughter in order to spare him additional grief.

Unknown to Sean, the baby Theresa had been found after the explosion by his cousin Black Tom Cassidy, who, suspecting that she might be a superhumanly powerful mutant as he and her father were, decided to raise her secretly himself. Theresa's sonic powers, inherited from her father, emerged at puberty, and a few years afterwards Black Tom began enlisting her aid in committing crimes. However, Theresa's heart was never in a life of crime, and she pursued it only because the man who raised her, Black Tom, ordered her to do so. (Although Black Tom Cassidy is actually Theresa's cousin once removed, she called him "uncle" as a mark of respect.)

Eventually, Theresa accompanied Black Tom and his partner, the Juggernaut, to San Francisco to assist him in crimes. Here, employing the name Siryn, she battled the original Spider-Woman and several of the X-Men. This conflict ended in Black Tom's temporary capture. While in custody, Black Tom exonerated Theresa of responsibility for her crimes and wrote a letter to Sean explaining who she was. The X-Men brought Theresa back to their headquarters, where she was joyfully reunited with her father.

For a time Siryn joined the Fallen Angels, a band of young super humans, hoping to keep them out of trouble. Later, she lived for a while at Moira MacTaggert's Mutant Research Center at Muir Island. She eventually came to join X-Force, the mutant strike team organized by Cable. As a testament to her skills, Theresa was made the deputy leader of X-Force after Cannonball graduated to the X-Men. Recently she has become romantically involved with the masked mercenary Deadpool.

Height: 5 ft. 6 in.
Weight: 112 lbs.
Eyes: Blue
Hair: Reddish blonde

Strength level: Siryn possesses the normal human strength of a woman of her age, height, and build who engages in intensive regular exercise.

Known superhuman powers: Siryn is a mutant who has the ability to create various sonic effects through the combined action of her vocal cords and limited psionic powers which can only be used in conjunction with the sounds she produces. She inherited these powers from her father, who has the same abilities.

By focusing her sonic vibrations into an intense beam, Siryn creates what she calls her "sonic lance," which can shatter any known solid substance except adamantium and vibranium. Siryn can use her "sonic lance" at a lower intensity to stun people into unconsciousness without doing them injury.

Siryn can use the sonic vibrations of her scream to propel herself through the air. Although it might be expected that emitting sound waves from her mouth would propel Siryn backwards, since for every action there is an equal and opposite reaction, this is not the case. Siryn's psionic abilities cause the acoustic energy to flow behind her. Siryn's psionic abilities enable her while in mid-flight to temporarily stop screaming to speak or to employ her scream to perform other sonic feats.

Siryn can also use her sonic powers as a form of sonar (Sound Navigation And Ranging).

Siryn's vocal cords and the interiors of her throat and mouth are superhumanly tough, so that the intense pressures and constrictions that she produces do not cause her any injury. While using her sonic powers Siryn also automatically and psionically creates a force field that protects her body from the destructive effects of the sound she creates. The force field only protects Siryn against her own sonic vibrations and nothing else, and can only be activated when she is using her sonic powers.