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 died in an IRA bombing. With no means to contact Sean, his cousin Tom took care of Theresa. When Sean returned to learn of his wife's death, he was devastated. Before Tom could even tell him of the existence of his daughter, Sean lashed out at Tom with his sonic scream for not having taken better care of Maeve. While Sean flew away in anger, Tom fell into a chasm, breaking his leg as a result of the attack, which left him with a limp. Angrily, Tom swore to make Sean pay and vowed to never tell him about his daughter, raising her himself instead. Tom (now known as Black Tom) would raise Theresa as his apprentice in his criminal activities. While still a young teen, Theresa accompanied Black Tom and his then-partner, the Juggernaut, to San Francisco to assist him in his crimes. Here, employing the name Siryn, she aided them in a theft of vibranium, and 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.
Theresa soon moved to Muir Island, home of the research laboratories of Dr. Moira MacTaggert, a longtime ally of Professor Xavier. During an overnight visit to Xavier Institute, Siryn, Kitty Pryde, Illyana Rasputin, and a young Muir Island resident named Amp battled robotic duplicates of the Brotherhood of Evil Mutants after accidentally releasing them from the Danger Room. Later in the evening, Siryn engaged Kitty in a vicious fist fight after Amp convinced her that the young X-Man was trying to sabotage her chances with a young man named Dmitri. After the fight was broken up by Wolverine, the girls reconciled and left on friendly terms. While living back on the island, Siryn started a romantic relationship with the mutant known as Jamie Madrox. This version of Multiple Man turned out to be one of his many duplicates and not the original Multiple Man, leading to the end of the relationship. During this time Siryn and Multiple Man traveled to New York City. She and Madrox began a search to help the New Mutants find two of their members, Sunspot and Warlock, who had run away from the school. In the course of this adventure, Siryn and Multiple Man found Sunspot and met the association of street-level mutants known as the Fallen Angels. They were transported, with the Fallen Angels, to Devil Dinosaur's planet. They then traveled to the "Coconut Grove", the home planet of the alien Ariel. While there, her superhuman powers were neutralized, and she was made a captive by Unipar of the "Coconut Grove" planet. She regained her superhuman powers and freedom, and returned to Earth. She decided to remain in the Fallen Angels for a time to help reform their criminal members. Later, back on Muir Island, she fell under the mental control of the Shadow King. Before long, she was freed from the Shadow King's control.
Siryn becomes a core member of X-Force, a paramilitary superhero team and for a time was its field leader. As a member of X-Force, her teammate Warpath develops a romantic interest in her while Siryn begins an on-going flirtation with the mercenary Deadpool. Deadpool and Siryn initially became involved when the two met while fighting Black Tom Cassidy and the Juggernaut. Though Banshee warns Siryn that Deadpool was a madman and a murderer, she found herself quite taken with him, even looking past his horrific appearance. Due to this kindness, Deadpool becomes infatuated with Siryn and often sat outside her window as she slept in the X-Mansion. At first Siryn is unaware of this, but she is comforted in knowing that he is watching over her. Over time, Siryn became the voice of reason and sanity in Deadpool's ear, attempting to sway him to the side of good. She assists him in some of his missions, such as when Deadpool felt he needed some blood from the Hulk in order to cure problems with his own healing powers. In the end, however, Siryn becomes overwhelmed with balancing her duties with X-Force and the demands of Deadpool's somewhat faulty sanity. She promises him that she would consider progressing their relationship if he continues on the path of good, and once she has her X-Force responsibilities secured. Warpath continues to seek Siryn's affections, and makes his dislike for Deadpool clear during a visit to the baseball diamond used in the movie, Field of Dreams. Siryn confronts varied adversaries during her time in X-Force. She helps put down a coup in the other dimensional realm of Asgard. She confronts the forces of Onslaught and the interdimensional media mogul known as Mojo. She even battles S.H.I.E.L.D. forces on more than one occasion. She also confronts more down to earth problems, particularly alcoholism. Warpath helps her to cope with her drinking problems, and as of recent issues of X-Factor she is leading a sober lifestyle. She becomes a vital part of the team, as opined by an aspect of Charles Xavier, which saves the entire team from a near-death experience. She leaves when Feral, a former teammate turned villain, slashes her throat, destroying her vocal cords and rendering her temporarily mute and powerless.
Thanks to the aid of Deadpool, Siryn has since fully healed from her vocal cord injuries. Deadpool tracked down Wolverine and stole a sample of his blood to heal her. Siryn joins the Paris branch of the international mutant task force X-Corporation, where she reunites with old friends such as Multiple Man, Rictor and Cannonball, and works for the first time with M. Their first mission does not go well and Siryn has to deal with the loss of another friend, Darkstar.
Siryn leaves X-Corp to become a member of X-Factor Investigations. Siryn, as part of her initial duties for X-Factor Investigations, investigates the murder of a woman who died in a movie star's penthouse, on behalf of the victim's sister. A rival firm, Singularity Investigations, represented the actor with Damian Tryp, Jr. as the defense counsel and to prove the star innocent. When Siryn and X-Factor Investigations manages to incriminate the star, and to thwart SI’s assassination attempt on Rictor, Tryp Junior is so infuriated that he ambushes Siryn and beats her, leaving her to die in an alley. She was later rescued by Rictor. Siryn was in a state of denial regarding her father's death, holding to the fact that many X-Men have been believed dead only to later return alive. Due to a love triangle with fellow teammates Multiple Man and Monet St. Croix, Siryn's relationships with the X-Factor members are strained. During a night of drinking on Jamie Madrox's part, Siryn ends up sleeping with Madrox, and St. Croix with his dupe. Both women are unaware that this had occurred, and are both hopeful of continuing a romantic relationship with him. Upon reabsorbing his dupe, Madrox is reluctant to tell the ladies, fearing for his life. Siryn resolves her differences with both Madrox and St. Croix, and the two women develop a budding friendship and partnership on later missions. Theresa and Monet are sent to retrieve two famous child singers (who specialize in mutant hate songs) from their parents, thanks to a court order from their grandparents. The two end up being ambushed, knocked out, and kidnapped by the children's mutant bodyguards, Solo and Clay. They awake imprisoned, (Theresa is tied up with her mouth covered by duct tape to prevent her from using her powers), but they both escape and return the children to their grandparents. It is revealed that the entire mission was a ruse by Josef Huber to lure the women away from their X-Factor teammates. Both Monet and Siryn's mental-based powers negatively affect Huber's mind-control abilities.
World War Hulk
Siryn answers the Stepford Cuckoos' call for help when the Hulk assaults the X-Mansion in "World War Hulk".
Theresa assists Rictor's infiltration of the Purifiers, and accompanies M and Strong Guy, along with the rest of the X-Men, to rescue the mutant baby and X-Force. She later receives a call from the mutant Peepers, who is being pursued by Predator X, but Theresa arrives too late to help him and finds what is left of his corpse. Siryn prays at the comatose Jamie's bedside when he awakes, and takes part in the X-Men's assault on the Marauders' base with Monet and Guido, taking on Exodus in the battle.
Divided We Stand
Siryn reveals that she is pregnant with Jamie's child, and after talking with Monet she reveals she intends on keeping it. She has not yet told Jamie but when she tries, he mistakenly believes that she wants to quit X-Factor. Monet later makes Siryn realize that she still loves Jamie. Theresa then tries to tell Jamie again but Arcade makes his presence known in Mutant Town so X-Factor split up to deal with him and help the residents. In X-Factor #10, Jamie Madrox awakes to learn that he and one of his duplicates slept with Theresa and Monet. Although he is initially uncertain as to which was which, he comes to conclude with near certainty that it was he that slept with Theresa, and the dupe that slept with Monet, though this does little to assuage Theresa and Monet's anger. Theresa later discovers that she is pregnant, and Jamie eventually learns about this.
In X-Factor #39, Theresa goes into labor. In the midst of it, she proposes to Jamie, who accepts. She gives birth to a boy, and they name him Sean, after her father, whose death she finally accepts. Just hours after his birth, however, Sean, much to the horror of Jamie, Theresa and X-Factor, is absorbed into Jamie's body as Jamie holds him, completely against Jamie's will. Jamie later says that Sean was an "infant dupe", and absorbed into him as a dupe would, but a grieving Theresa is only filled with rage toward Jamie.
Assuming Leadership and Departure
After the pregnancy, Madrox leaves the team and Terry takes charge, though she is sinks into sadness because of the recent events. Following Madrox's return to present time, he offers to leave the team headquarters in Detroit and restart a branch with Guido in New York. Shortly after this, she disbands the Detroit team, bringing most of the members to return the New York branch. She later runs into Madrox's reverend dupe who helps her cope with her depression and rekindles her relationship with Deadpool. After coming to terms with his death and reconciling her grief, Siryn has taken her deceased father's codename as her own and begins calling herself Banshee in honor of his memory.
Powers and Abilities
Siryn possesses sonic powers similar to those of her father Banshee, although she is able to use her powers in ways that he apparently could not.
Audiokinesis: 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.
- Sonic Scream: Siryn can cause her vocal cords to vibrate over a wide range of sonic frequencies and produce a volume of sound ranging up to at least 140 decibels. Siryn has thus the ability to shatter solid objects with a sonic scream
- Flight: Siryn can use the sonic vibrations of her scream as a carrier wave.
- Sonic Lance: By focusing her sonic vibrations while screaming into an intense beam. This is used as a sort of "battering ram" that can stun or even destroy most solid objects.
- Sonar: can use her sonic powers as a form of sonar. By sending out a precise, pure note and listening to the returning, altered wavefront of the note, she can evaluate her surroundings in total darkness with varying degrees of resolution.
- Sonic Null Field: can project a field of silence in a area making no sound reach outside the area or no sound outside the area in.
- Sonic Amplification: can amplify sound carrier waves allowing her to hear conversations or noises from far off.
- Sonic Shield: can tighten sound waves around herself or others near her, making a barrier against outside attack.
- Vocal Disorientation: ability to change someone’s balance, by affecting their equilibrium using her sonic scream.
- Vocal Trance: ability to subtly influence peoples subconscious mind by changing the tones and vibrations of her voice using sonics, the effects tend towards blatant hypnosis
- Vocal Unconsciousness: ability to affect the fluid in someone’s ear causing the person to go unconscious.
- Siren's Song: ability to influence human beings with a certain pitch of her voice, causing them to fall in love with her without regard to gender or sexual orientation.
- Sound Immunity: Siryn's hearing and equilibrium are unaffected by the effects of her scream and similar abilities.
Superhuman Vocal Stamina: Her vocal cords are tremendously strong, so that she can produce sonic effects through screaming constantly for hours at a time without noticeably tiring or injuring her vocal apparatus. Siryn’s vocal cords and the interiors of her throat and mouth are also superhumanly tough, so that the intense pressures and constrictions that she produces do not cause her any injury.
Siryn has survived major injuries to her throat, although such injuries have left her temporarily unable to use her sonic powers. Excessive vocal cord strain can also cause her to temporarily lose her powers.
In other media
In the film X2: X-Men United, Siryn was played by Shauna Kain in a small cameo role. During William Stryker's attack on the mansion, her scream alerts the rest of the school that they are under attack. She was knocked out, but was rescued by Colossus. Her name (along with her father's) also appears on a list of names Mystique scrolls through on Stryker's computer while looking for Magneto's file. Kain reprised the role in X-Men: The Last Stand.