Adventurer
Interpol Inspector
Mutant Rights Activist
Private Detective
Teacher
Factor Three
Cassidy Keep, Ireland
Muir Island, Scotland
Excellent hand-to-hand combat skills with training as a police officer.
Note: Powers are susceptible of causing injury to the vocal cords from excessive strain.
Factor Three
Mutant Master
Proteus
Generation X
Uncanny X-Men
X-Men Vol. 2
While at Interpol, Cassidy was assigned a top secret mission, which required him to go under deep cover. While under cover, his wife discovered she was pregnant and gave birth in his absence to their daughter Theresa (Siryn). While visiting relatives in Northern Ireland, Maeve was killed by a terrorist bombing. No trace of Theresa was found and she was assumed dead. It was revealed years later that Cassidy's cousin Tom (Black Tom), knowing of Sean's powers, abducted the infant in hopes that she would have similar abilities.
Returning from his mission, Cassidy focused his grief of his wife's death into his career. He was never informed he had a daughter to spare him additional misery. The melancholy Cassidy left Interpol to work as a mercenary. Although never arrested, his activities did not go unnoticed.
Cassidy was approached by a representative of a mutant terrorist group known as Factor Three, called the Changeling. Cassidy refused, but was forced to join by the use of an explosive headband, which would be remotely detonated if he refused or attempted to remove it. He was code named Banshee by the groups leader, Mutant Master, and assigned to a human operative known as Ogre.
A mission for Factor Three in New York would bring Banshee into conflict with the X-Men. He and Ogre were captured. Professor Xavier freed Banshee from his explosive headband and he joined the X-Men against Factor Three, revealing their leader the Mutant Master as a member of the extraterrestrial Nox.
Cassidy would return to his Irish homeland, but later join Professor Xavier in a rescue mission of his original students, as a member of the all new all different X-Men.