Irish-born Sean Cassidy lived the adventurous life long before joining the X-Men. Once an Interpol inspector and later freelance secret agent, Sean often found himself knee-deep in dangerous situations. One such predicament involved Factor Three capturing Cassidy and employing the use of his "sonic scream" against the X-Men! Banshee helped defeat the villains, however, and befriended the X-Men in the process-making him a top candidate for Professor X's new team of uncanny X-Men!