Siryn (Theresa Rourke Cassidy) is a Marvel Comics superhero, associated with the X-Men. Like her father, the X-Men’s Banshee, Siryn 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. Siryn was raised by Banshee’s cousin and arch-nemesis Black Tom Cassidy without Banshee's knowledge. By her early teens, she left Black Tom and joined the X-Men offshoots X-Force and X-Factor.
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 chords. 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. Siryn once utilized this ability on Spider-man, forcing him to divulge the truth about the Decimation of the mutants.
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.