Banshee, Sean Cassidy, was not an original member of the X-Men as depicted in the movie; X-Men: First Class. Instead he led a far more intriguing and interesting life.
Cassidy first appears in January 1967 in X-Men #28 and was actually a foe of the X-Men, though it would be later explained that he acted that way under coercion. It would not be until the new X-men team of Storm and Wolverine in Giant-Size X-Men #1 of May of 1975 that Cassidy would join the team and become an X-men.
Cassidy is heir to Cassidy Keep in Ireland and born into a well to do life. As a young man he joins Interpol as an Inspector and marries Maeve Rourke who gives birth to their daughter, Theresa Cassidy. Sean powers manifest and he develops his sonic blast. Sean is away at the time of his daughter’s birth and does not even realize he has a child. Before she can tell him, Maeve is killed in an IRA bombing and as Sean is away as an agent of Interpol, his cousin Tom takes charge of the child. When Sean returns he lashes out at Tom over his wife’s death with his sonic scream, crippling his cousin. Angered, Tom decides to get revenge on his cousin and keeps the existence of Theresa a secret and raises her as his own.
Cassidy soon leaves Interpol and falls under the control of a group called Factor Three who places a headband containing explosives on him and forces him to perform missions for them. One of these missions takes him to New York where he encounters the X-Men. It is at this time that he takes on the name of the Banshee. Professor Xavier disarms the headband and frees Cassidy who then aids the X-Men in defeating Factor Three.
Years later Cassidy joins the X-Men and was present for many of the turning points in the history of mutant kind, including the first appearance of the Phoenix and their first encounter with the alien race, the Shi’ar. While with the X-men Cassidy meets Professor Xavier’s girlfriend Moira MacTaggert and falls in love with her. During this time, his daughter Theresa begins to develop her own sonic powers and takes the name Siryn. She begins to help her Surrogate father Black Tom Cassidy with his criminal activities. Banshee soon begins to lose his powers as his vocal cords are damaged in battle. He stays on Muir Island with Moira MacTaggert to heal and begin to reconcile his relationship with his daughter Siryn.
As Banshee heals he becomes co-headmaster of the Massachusetts Academy of the X-Men where he teaches the young students of Generation X, along with Emma Frost in the use of their powers. The Academy eventually closes it’s doors as the students abandon the school and their teachers. It is also at this time that his love, Moira MacTaggert dies at the hands of the shape shifter Mystique.
In a depression, Banshee forms a separate group of Mutants who disagree with the X-Men and methodology and membership practices. With this new group called the X-Corps, Banshee goes to battle with Mystique who was masquerading as one of his team. Mystique stabs Banshee in the throat before the X-Men can stop her. As he is recuperating from the attack and depressed even further in how his life has turned, Banshee attempts to stop the Blackbird from colliding with an airplane full of innocent people. Not healed sufficiently, his sonic scream could not stop the Blackbird which flies through Cassidy and collides with the plane. Nightcrawler and Wolverine find Banshee’s remains in the wreckage.