[4] Theresa Cassidy debuted as a vilain in Spider-Woman #37 (April 1981), created by writer Chris Claremont and artist Steve Leialoha.

Maeve later dies in an IRA bombing, and with no means to contact Sean, his cousin Tom takes care of Theresa.

[13] Due to her inexperience, she performs poorly in battle and is defeated by Spider-Woman again during Black Tom's temporary capture.

[15] Theresa moves to Muir Island, home of the research laboratories of Dr. Moira MacTaggert, a longtime ally of Professor Xavier.

[volume & issue needed] During an overnight visit to Xavier Institute, Siryn, Kitty Pryde, Illyana Rasputin, and a young Muir Island resident named Amp battle robotic duplicates of the Brotherhood of Evil Mutants after accidentally releasing them from the Danger Room.

Dazzler is impressed with Theresa's rendition of "Amazing Grace", that she asks the young mutant to act as a backing vocalist for her concert.

[19] Siryn and Multiple Man find Sunspot and meet the association of street-level mutants known as the Fallen Angels,[20] and they are all transported to Devil Dinosaur's planet.

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.

[volume & issue needed] She leaves when Feral, a former teammate turned villain, slashes her throat, destroying her vocal cords and rendering her temporarily mute and powerless.

[volume & issue needed] Siryn joins the Paris branch of X-Corporation, claiming to atone for her father's sins for his failed X-Corps idea, where she reunites with Multiple Man, Rictor and Cannonball.

[36] During the Superhuman Civil War, Theresa opposes the registration act and is tasked with contacting more high-profile superheroes about the Decimation.

After speaking with Spider-Man, X-Factor gets into contact with Quicksilver and Cyclops' team of X-Men, demanding they answer why the reason behind the Decimation was kept secret and defy the registration act.

[40] Theresa resolves her differences with both Madrox and Monet, and the two women develop a budding friendship after a shopping trip to Paris and partner together on later missions.

[43] Theresa assists Rictor's infiltration of the Purifiers[volume & issue needed] and accompanies the X-Men to rescue the mutant baby and X-Force.

[volume & issue needed] She later receives a call from Peepers, who is being pursued by Predator X, but Theresa arrives too late to help him and finds his remains.

[44] Siryn prays at Madrox's bedside when he awakes, and takes part in the X-Men's assault on the Marauders' base with Monet and Guido, fighting Exodus in the battle.

[49] Following Jamie's return to the present, he offers to leave the team headquarters in Detroit and start a branch with Guido in New York.

Theresa returns home to Ireland where she runs into Reverend John Maddox, one of Jamie's dupes, who helps her cope with depression.

[51] She is rescued by Layla Miller and Shatterstar, and they teleport back to America and rejoin X-Factor in the final battle with the Mutant Response Division.

[57] Theresa and the rest of X-Factor later become the target of various mystical canine and feline deities who attempt to capture Wolfsbane and her unborn baby.

[60] After Havok and Polaris rejoin X-Factor, Theresa is present when Layla reveals Jamie has come back from the dead but not by her doing.

[62] Wolverine learns that a woman matching Theresa's description has been attacking and killing people in Ulster County, New York.

Jezebel advises them the apocalypse is coming and Theresa will have to make a tough choice sometime within the near future: to become part of a race of new gods or perish with the rest of humanity.

[66] Theresa goes back to her room, where she continues to converse with Sean about the Morrigan and if he is real or some kind of hallucination when Layla enters, who reveals she knows what is going on.

[70] She has shown a degree of resistance to sounds and vibrations that would be harmful or painful to normal humans and mutants without sonic-based powers.

[72] She can 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.

[66] She is able to revert Jamie to his human form and absorb laser blasts from a gun that can destroy Layla Miller's force field.

[76] Deirdre Kaye of Scary Mommy called Theresa Cassidy a "role model" and a "truly heroic" female character.

[77] Dalton Norman of Screen Rant included Theresa Cassidy in their "10 Best Marvel Characters Who Made Their Debut In Spider-Man Comics" list.

After the X-Men's death on Krakoa, Beast hastily assembles a mutant team to combat Count Nefaria and his Ani-Men.