Theresa Cassidy was born in Cassidy Keep, County Mayo, Ireland to Maeve Rourke and Sean Cassidy. Shortly after he was married to Maeve, Sean had to leave on an INTERPOL mission, and he could not maintain contact with his wife, who found out she was pregnant after he left. Sean was gone for a pretty long time, more than a year, and she gave birth to baby Theresa in his absence. Unfortunately, Maeve was also killed in a terrorist attack before her husband returned, leaving Sean’s jealous cousin Tom--who also loved Maeve--to care for Theresa until Sean came home. When Sean returned, he was devastated to find out his wife had died, and lashed out by attacking Tom with his sonic scream, then flew off. Therefore, Tom was prevented from telling Sean about his baby daughter, and Sean disappeared from Cassidy Keep for years, unaware that there was a child waiting for him in the castle. Sean would later join the X-Men as Banshee, and Tom was getting involved in profitable crime, later to appear as the villain and international black market criminal Black Tom.
Tom raised Theresa like he would his own daughter, as he loved the child dearly, but told her that her father was dead. Theresa was sent to boarding school when she was 12 so she wouldn’t find out about the illegal activities Tom was involved in. When she was 13, Theresa started drinking, and it developed into alcoholism. Tom was arrested by INTERPOL and sent to prison when Theresa was 15, exacerbating her drinking problem. But Tom broke out of prison after meeting Cain Marko aka the Juggernaut, X-Men founder Charles Xavier’s stepbrother, and they took Theresa to San Francisco on a crime spree, where they ran into Spiderwoman and the X-Men. When the rumble was over, the criminals were defeated and Theresa was unconscious. Tom told the X-Men his cousin wasn’t responsible for any of their crimes, and they took her to meet her father, who was overjoyed to have her in his life.
She lived at Muir Island with her father and his lover, Moira MacTaggert, for some years, where she met Jamie Madrox, aka Multiple Man, and the two of them enjoyed a close relationship for a time, including a stint in the Fallen Angels after being assigned to bring a runaway Sunspot back to the New Mutants. She eventually broke up with Jamie, however, but it later turned out he wasn’t really Jamie at all, but a renegade duplicate. When the Shadow King attacked Muir Island, Siryn was among the mutants he put under mind control and forced to behave in ways she wouldn't like to repeat, and so Theresa was left rather traumatized and scarred after the attack. Following that episode, Theresa traveled to the United States to stop her uncle and Marko in the act of yet another criminal event, where she met X-Force, and agreed to join them.
While in X-Force, Theresa became good friends with her teammate James Proudstar aka Warpath, who developed feelings for Theresa and discovered that she was an alcoholic. On Cable's advice, James confessed to Theresa that he liked her, and also confronted her about her drinking. Theresa was, at first, not accepting of his attempt, but soon asked him to come with her to Cassidy Keep so she could "straighten [her] head up a bit." That visit--X-Force #31--turned into two weeks in which James, in cooperation with Juggernaut, helped Theresa to confront Black Tom about their past, an encounter which led to her giving up the drink. She appreciated James's friendship, but he cared about her more than she realized, and she wasn’t interested in a relationship with him until after he met Risque, who loved him but betrayed him.
Although Black Tom was arrested after his latest meeting with Theresa, he didn't stay in custody, and so Theresa and Sean were soon to be seen teaming up with the mercenary Wade Wilson, generally known as Deadpool, to track him down and get him stabilized. Thus began Theresa's friendship with Deadpool, who also developed feelings for her. After Sam Guthrie left X-Force and joined the X-Men, Theresa became the deputy leader of the team, which meant very little until Cable and Domino left X-Force, making Siryn the new leader.
Siryn led the team for some time, dealing with a bit of disrespect from a few of her teammates who couldn't see the logic in Siryn=leader, but nonetheless keeping the team together and safe. Domino returned to the team at the same time as Jesse Aaronson invaded, and relieved Siryn of the leadership responsibility. In issue #90, while X-Force was battling the New Hellions, led by Jesse Aaronson's brother, Christoper, Theresa was attacked by former teammate, Feral, and her vocal chords were damaged so badly she was left unable to speak or use her powers. She then left the team, and moved in with her cousin (on mother's side), Victoria Donnelly. However, that didn't last long before she was possessed by the mind of some weird old woman named Ophelia and, despite being still depowered, flew off.
In Wolverine #154-5, Siryn was handed over by Deadpool (apparently while unconscious) to a shady group called the Watchtower, who used Wolverine's healing factor to repair the damage to Theresa's vocal chords, thereby restoring her voice and powers. She was pleasantly surprised, to say the least, to wake up and find that she could scream again, but wasn't entirely comfortable with the means by which Deadpool had helped her. She later appeared in an issue of Deadpool, trying to go out on a date with Wade but foiled by the jealous Vanessa. While waiting at the Botanical Gardens for Deadpool, Theresa was attacked (and, by the Deadpool writer's and editors' logic, quickly and unqualifiedly subdued) by Vanessa in Deadpool's form. Theresa then went to Wade's apartment and, thinking that it had been he who had attacked her, bitch-slapped him and walked away.
Theresa was thus consigned back to Comic Limbo until after the conflagration of Banshee's mutant police program, X-Corps, when she joined the X-Corporation's new strike force in response. In her own words, she was there to "atone for [Sean's] sins." That stint consisted mainly of an ugly incident in the Euro Tunnel, following which she slipped back into obscurity for a while more. By that point, X-Force as she'd known it was gone and cancelled, but then Cable reassembled the team for a limited series that ended up with him sacrificing himself into multi-dimensional chaos during a battle with the Skornn. Siryn was not involved with that particular battle, but she was not unaware of events. Following Cable's sacrifice, Theresa put on a butt-ugly costume and teamed up with Sam "Cannonball" Guthrie and found Deadpool on Cable's haven island, Providence. Theresa and Sam worked with Deadpool, using Forge's technology, to rescue Cable's essence and bring him back to the Earth of their dimension.
The adventure with Cannonball and Deadpool spread over the House of M event, which resulted in the Decimation. In response to the fallout of the Decimation, Jamie Madrox--the real one--formed X-Factor Investigations, a mutant detective agency in Mutant Town, which Siryn joined as an operative. She keeps getting shot, stabbed, beaten up, and otherwise roughed up in the course of her work with them, so of course she loves her job. While she was recuperating from a beating at the hands of Damian Tryp, of the rival agency Singularity Investigations, Cyclops visited headquarters and told her the news of Sean Cassidy's death in battle. Theresa appears firmly entrenched in denial, insisting that her father is simply in hiding. She still hasn't yet acknowledged that he's dead. She is also, reluctantly but surely, becoming good friends with her teammate Monet St. Croix.
