Sean Cassidy, also known by his codename Banshee, was an American [[Mutant] who was well known for his participation in the Cuban Missile Crisis.
Before being confronted by Charles Xavier and Erik Lehnsherr, Sean was attempting to flirt with a woman. He readily accepted the offer to join Division X when asked and shortly after meeting his partners, was attacked by the Hellfire Club and saw Sebastian Shaw murder Darwin. After the closure of Divison X, Sean, along with the rest of his teamates, relocated to the X-Mansion. There Sean took the codename Banshee and honed his abilities under the tutulege of Xavier. Sean attended Xavier's School for Gifted Youngsters shortly after the Cuban Missile Crisis, and after graduating, left to live his own life. However, he was not safe, as he was captured and killed by Trask Industries.
Cassidy possessed Acoustikinesis, allowing him to scream at sonic levels and shatter glass. His scream was so powerful, that if given the proper suit, he could fly and create sonar waves underwater.