History: Born in Ireland, Sean Cassidy became involved in protecting the innocent long before the erruption of mutants in the world. After joining Interpol, Sean had a face-to-face with a couple future adversaries, and threats that he had no idea could be so large. One of which was Arkady Rassovich, the man who became known as Omega Red. He also had a meeting with Magneto, though he didn't realize it at the time (as this was before the Cape Citadel incident). His wife, Maeve, died around the same time, leaving a baby daughter named Theresa to be in his care, but Sean's cousin, Thomas, took the baby and raised her by himself, emersing her in his criminal acts.

Following this, Sean set out to New York City, where he joined the police force and had an interesting run-in with Emma Frost and the man known as the Black Beast. Sean soon became even more involved in the affairs of other mutants, namely when he first met the X-Men. Then, he was considered a villain, since he was working with Factor Three, a mutant terrorist organization. With a bomb strapped to his head, he had no choice but to follow orders play the part of an evil mutant. Eventually, he was freed by the X-Men, only to be kidnapped again by Factor Three later, only to be freed once more by his newfound friends.

Becoming an X-Man some time later, along with several other mutants from around the globe, this new incarnation of the X-Men battled the living island known as Krakoa, freeing the original X-Men. He stayed with the team until he was injured in a battle with the mutant known as Proteus, at which point he stayed on Muir Island and met Moira MacTaggart. The two soon became involved romantically with one another. Banshee never became an official member of the X-Men after that, but did act as a sort of reserve member numerous times, including a battle against the freakish cyborgs known as the Reavers, and helping a number of Morlocks following the Mutant Massacre.

Returning to the team once more for a time, Banshee soon found himself surrounded by imposters of the X-Men, who turned out to be members of the collective race known as the Phalanx. Intent on absorbing and catalogueing the next generation of mutants, the Phalanx attacked several youngsters, which led Banshee to require the assistance of villains Sabretooth and the White Queen, though the latter was a bit more reformed at this point. The battle that ensued, leading to the death of Blink, one of the young mutants captured by the Phalanx, also led to the formation of Generation X, the next generation of X-Men. He is now one of three headmasters of the Massachusettes Academy, along with Emma Frost and her sister, Adrienne, while they train these young mutants, and run a respectable private school as well. For the most part, he's retired from the superheroics and is thought of more as a teacher and guardian.