A hero/villain published by Marvel Comics. Banshee first appeared in X-Men #28.

Sean Cassidy was born the heir to Cassidy Keep in Ireland where he was raised. His position with in his family along with his romance of a woman named Maeve Rourke earned Cassidy the emnity of his cousin, Thomas Cassidy. The two competed for Maeve's affection, which was won by Sean.

During his teens, Sean discovered that he was in fact a mutant with the ability to create special effects when he screamed. The sound waves that Sean produced allowed him to fly, to project them in a tight area as a weapon, and to knock people unconcious. Sean kept his abilities secret from all of his family and even his employers at Interpol.

Sean and Maeve were married, while Sean served as an inspector within the intelligence agency Interpol. While Sean was away on an extended mission that required him to not be in contact with his family, Maeve gave birth to their daughter. Soon after, Maeve was caught in a terrorist bombing and killed. Her relatives believed that Theresa, the daughter, had also been killed. In fact, she had been saved by Sean's cousin, Thomas, now known as Black Tom who raised the child. Sean returned to find that his wife was dead, and his relatives did not tell him of his daughter to save him further grief.

Sean continued to serve in Interpol for a while, eventually retiring. He worked as a freelance operative for a time, eventually findng himself on the wrong side of the law. He was never caught for his crimes though he did clash during this time with the mutant group, the X-Men. Sean had begun to use his abilities while calling himself Banshee after the legendary creature. Cassidy decided to go straight and quit being a criminal, but was forced for a time to work with the criminal group Factor Three, when they captured him and outfitted him with a headband containing explosives that they would use if he did not obey. Banshee was helped by the X-Men who help him get free of the headband and then he helped them defeat Factor Three.

Banshee dropped off the scene for many years until he was contacted by Charles Xavier after his students disappeared. Gathering with a new group of recruits, Banshee and the others rescued the original X-Men from the clutches of Krakoa, the Living Island. After this, the new recruits decided to continue on as X-Men.

During this time, Cassidy met and fell in love with Charles Xavier's friend, Moira MacTaggert. MacTaggert worked with the X-Men for a time, but later returned to her research station on Muir Island off the coast of Scotland.

A series of adventures brought X-Men to Japan and pitted them against the villain Moses Magnum. Magnum had constructed a sonic cannon and threatened to use it to sink a number of islands in the Japna chain if he was not paid a ransom. The X-Men engaged him in battle, but not before he could set off his device. Banshee used his mutant powers to match the cannon with opposing sonic waves cancelling out the destructive power of the cannon. In doing so, Banshee pressed his abilites beyond their limits and seemingly damaged his vocal cords. After they healed, Banshee was able to speak, but maintaining his sonic scream was beyond his abilities.

Banshee retired to Muir Island and spent time with MacTaggert. He did come out of retirement for a short time to lead a group of heroes to rescue relatives of the X-Men from the clutches of the assassin Arcade. During this time, Banshee also learned of the existance of his daughter Theresa who had gained powers similar to his and had adopted the name Syrin.

With the help of the Morlock healer, Cassidy's ability to use his sonic scream was returned to him so that he could aid in the defense of Muir Island from an attack by a group of cyborg warriors known as the Reavers. Cassidy returned to X-Men for a time and later became the headmaster of the Massachusetts Academy, training a new generation of mutants who later became known as Generation X with the help of former White Queen of the Hellfire Club, Emma Frost.