Banshee is a mutant who can warp reality through the use of sounds produced by his voice. He can use this power to fly, to shatter solid objects, to place people in a temporary trance, or to cause people to fall unconscious.
History
Sean Cassidy was both the heir to a small fortune and a castle in Cassidy Keep, Ireland. In his youth, he married Maeve Rourke, and took a job at Interpol. While Cassidy was away on a long mission, his wife discovered she was pregnant and gave birth to their daughter, Theresa Cassidy. With Cassidy still on assignment and unaware of his daughter's birth, Maeve was killed by a terrorist bomb in Northern Ireland. Cassidy's disreputable cousin, Black Tom Cassidy, rescued Theresa, secretly planning to raise her himself. When Cassidy returned, he was devestated to hear of Maeve's death. Those who knew that Theresa had been born decided not to tell Cassidy that he had had a daughter in order to spare him further grief.
Cassidy left Interpol to become a freelance operative. The villainous Changeling discovered him through the group Factor Three, and invited him to join the organization. Cassidy was appalled upon learning of Factor Three's goals and adamantly refused. However Factor Three captured him and placed a headband containing explosives around his head to force him to obey them. Code-named The Banshee after the spirit from Irish mythology, Cassidy felt forced to obey Factor Three's commands, and performed various criminal missions for the organization. On a mission in New York City, Banshee encountered the mutant team of superheroes the X-Men. Professor Charles Xavier was able to use his telepathy to disarm the headband and remove it, and Banshee helped the X-Men defeat Factor Three.
A few years later (Giant-Size X-Men #1), Xavier approached Banshee to join his second group of X-Men, and Banshee accepted. There he met and fell in love with the X-Men's ally, Dr. Moria MacTaggert. However, Banshee lost his powers in battle for a time and left the X-Men.
Banshee later learned of his daughter Theresa's existence. Theresa had developed sonic powers of her own, which she used under the alias of Siryn, and felt obligated to Black Tom, who had raised her, to assist him in his crimes. But Siryn reformed after the pair were defeated by the X-Men and she was reunited with her true father.
Banshee is now the co-headmaster at the Massachusetts Academy, where he teaches the young mutants of Generation X in the use of their powers alongside Emma Frost.