Like her Father, Theresa has a sonic power. She is capable of emitting sonic vibrations of varying frequencies and volumes to produce a multitude of effects. She can release it as a concussive blast capable of mass destruction, shattering metal, disrupting electronic equipment or use it to counter other forces such as a punch or a kick. Theresa can scream out a high frequency sonar in order to locate objects in darkness. She can release a scream which can cause temporary pain and disorientation or even severe nausea, and temporary hearing loss. Theresa can fly by riding existing sound waves in the air. This allows her to use her flying while talking or using her power. In an environment lacking any sound at all, she can fly be generating the sound herself.

Theresa also has much stronger vocal chords than her father, allowing her to put greater strain on her abilities without severe damage to them and allows her to use them more often, usually on a day to day basis. Theresa is still learning about her abilities and there may be facets which she has yet to discover.

Theresa was born the daughter of Maeve O'Rourke and Sean Cassidy. She was born while her father was away on a mission, who did not know Maeve was pregnant, so he had no idea he had a daughter. The mother was killed in an IRA bombing and it was assumed that the babe died as well, but she didn't and was rescued by her cousin, Black Tom Cassidy. To spare Sean grief, those who knew of the babe decided not to tell him about her. Black Tom kept her out of jealousy of Sean. Despite his evil, misguided life, he loved Siryn like his own daughter and cared for her deeply. After Black Tom's release from prison, he attempted to make Theresa follow him in his criminal career.

Together Tom, Cain Marko, and Theresa went to San Fransisco, and performed a series of robberies. They were eventually stopped by the X-Men, and after they were defeated Tom said Theresa had nothing to do with the crimes. He handed her a letter to give to her father, which explained that she was his daughter.

After the reunion with her father - who was almost died of happiness on the spot after finding out he had one - she decided to live on Muir Island. Theresa lived for a long time on Muir Island, developing a relationship with Jamie Madrox, also a mutant. Both them joined the Fallen Angels in pursuit of Boom Boom, but left the team after some adventures. After an attempt to resurrect Moira's son, Kevin (Proteus), was stopped, the Shadow King struck and almost defeated the X-Men, destroying most of Muir Island in the process. She left Muir in order to stop a terrorist attack on the World Trade Center by Black Tom and Juggernaut. There she met and joined X-Force and has been with them ever since.

As Jamie left to join X-Factor, it was discovered that the one who loved Siryn wasn't the real multiple man, but one of his duplicates who was trying to be independent (a sort of mutation of the mutant). The real Jamie was still pretty cold to Theresa.

It wasn't until later that they worked things out and Jamie realized his true love for her. When Sam Guthrie (Cannonball) was graduated from X-Force to become part of the X-Men, she stepped in as Deputy Leader of the group.