While Sean Cassidy (her father Banshee) 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 diee in an IRA bombing.

With no means to contact Sean, his brother 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.

Sean fliew away in anger, while Tom fell into a chasm, breaking his leg as a result of the attack, which leaves him with a limp. 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) trained 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. Here, employing the name Siryn, she aided them in a theft of vibranium, and battled the original Spider-Woman. Due to her inexperience, she performed poorly in battle and was defeated by Spider-Woman again during Black Tom's temporary capture. Initially believing her to be seriously hurt, Black Tom resolved to break all association with her so that she will not be exposed to further danger. While in custody, he exonerated Theresa of responsibility for her crimes, and wrote a letter to Sean explaining who she is. The X-Men brought Theresa back to their headquarters, where she was joyfully reunited with her father, who moved her to Muir Island for her protection and further training.

Powers: Siryn is a mutant who possesses sonic powers similar to those of her father, Banshee, although she is able to use her powers in ways that Banshee apparently cannot. Like her father, she has the ability to emit high-decibel "sonic screams" to create various sonic effects with her vocal cords. She can also vary the pitch to deflect projectiles, and she even used it to deflect lasers once. She possesses an enhanced sense of hearing and can use her voice for echolocation to detect objects. While screaming, Siryn is able to fly at the speed of sound. Siryn has especially potent sonic attacks. She can shatter objects; project concussive blasts of sonic force (a "sonic lance"); or cause immediate pain, nausea/vomiting, and unconsciousness. She has shown a degree of resistance to what would be harmful and painful sounds and vibrations to normal humans or mutants without sonic-based powers.[56] She also possesses psionic powers to create a force field that protects her against her own sonic vibrations to assist her ability to fly, and to help direct her sonic powers.

She is vulnerable to losing the ability to use her power effectively by overstraining and overusing her sonic screams or injury to her throat and vocal cords. She also lacks fine control of her abilities when in extreme distress: screaming in pain during her pregnancy, she unwillingly shattered every window in the complex, causing the medical staff to writhe in agony.

Unlike her father, Siryn is able to influence and control other human beings with her sonics, as well as cause them to enter a hypnotic state. Siryn can also cause the intended person to fall in love with her without regard to gender or sexual orientation and carry out her wishes and commands, like mythical Sirens.

In addition, she can also prevent others from hearing external sounds by a certain vocal pitch so that she can secretly deliver messages to them.