- "I’m going to be... Banshee."
"Why do you want to be named after a wailing spirit?"
"You may want to cover your ears..."
- ―Sean Cassidy and Hank McCoy
Banshee (born Sean Cassidy) was a mutant who had the ability to create high-powered sonic waves with his voice.
Banshee is first seen hitting on a girl in an aquarium. However, she doesn't accept him, and actually finds him comical. In a final attempt to gain her affection, Banshee uses his ability to scare away all of the fish. It doesn't work, and at this point, Charles Xavier and Erik Lehnsherr appear, and convince Banshee to join the X-Men off screen. Later, Banshee is seen at the government facility with the rest of the X-Men. Banshee, Darwin, Angel, Beast, Mystique, and Havok are all seen getting to know each other, while also being taunted by several government agents who don't accept Banshee and the rest of the mutants. Then, around this time, Sebastian Shaw and the rest of The Hellfire Club attack the facility and kill several of the agents. Shaw, Azazel, and Riptide corner the young members of the X-Men. Shaw tires to convince them all to join him, and, much to the X-Men's horror, Angel does. Darwin, in an attempt to save Angel and kill Shaw, pretends to join The Hellfire Club. However, this fails, and Shaw kills Darwin by transferring Havok's energy into Darwin's body, causing Darwin's body to explode right in front of the X-Men.
Banshee is next seen with the rest of the X-Men, mourning Darwin. Xavier is hesitant about continuing to lead the team, but Banshee helps manage to convince Xavier to continue to lead them, telling him that he doesn't want to see Havok go back to prison. So, Xavier takes Banshee and the rest of the X-Men to his home, the X-Mansion. There, Banshee and the rest of his team train for combat against Shaw and the Hellfire Club. While standing out in the courtyard with Charles and Moira, Charles helps him focus his power and he shatters a glass window, Charles tells him to go at a higher frequency and it gives Moira and Charles nausea. Beast designs a wingsuit for Banshee, intending for Banshee to use his sonic screaming to propel himself through the air. At first, Banshee is nervous about using his suit, and falls out of a window in a failed attempt at flying. Later, however, Banshee is seen standing at the top of a satellite dish, along with Charles, Erik, and Hank. Reluctant to jump and try to fly, Banshee claims that he is afraid of dying, at which point Erik pushes him. Banshee screams in terror, which generates loud enough sound waves for him to fly, helping Banshee to gain confidence and master his new ability.
Banshee's ability of flight proves to be extremely useful during the fight against the Hellfire club later on, as he rises out of the water (after using his sonic screaming as sonar in order to locate Sebastian Shaw's submarine) and uses his scream against Tempest, disorienting her and saving Havok's life. Tempest flies after him though, and she burns a hole in his wing suit, causing Banshee and Havok to fall onto the beach. After this, Banshee and the rest of the mutants witness Erik expose Shaw's body and attempt to destroy all of the ships on the coast. Moira MacTaggert, however, tried to stop Erik by shooting at him. Erik blocked the bullets, but one rebounded and hit Charles in the back, resulting in his paralysis. When Banshee and the rest of the X-Men attempted to intervene, Erik used his ability to throw Banshee and the others backwards onto the beach. Then, after Erik and his newly formed Brotherhood of Mutants leave the beach, Banshee (along with Beast and Havok) run to Charles and Moira, who are both lying in the sand, as Charles is now paralyzed.
Erik mentions that Banshee, along with Azazel, Emma Frost, and Angel Salvadore, had died in between 1962 and 1973, and blames Charles for their deaths. He was among those experimented on and killed by Bolivar Trask. It is unknown if Mystique saw him on the autopsy reports.
Banshee's legal name, Sean Cassidy, appears on a list of mutant names that Mystique scrolls through on William Stryker's computer while looking for Magneto's file. Possibly some names of mutants Bolivar Trask experimented on.
- Acoustikinesis - Banshee has the mutant ability to create powerful sonic waves with his voice with which he could achieve various effects.
- Echolocation - Banshee can generate sound waves underwater that ricochet off of metal objects and then back at him.
- Nausea Inducement - By producing an extremely-high frequency, Banshee can make people sick and lose their coordination.
- Sonic Scream - Banshee has the ability to scream at sonic levels that are loud and destructive. His screams are deafening to nearby humans and can shatter solid objects, such as glass, or scare away animals.
- Sonokinetic Flight - Banshee has the ability to fly at subsonic speeds by screaming supersonic waves towards the ground, in order to propel himself through the air and ride via sound waves. When using this ability, Banshee could carry at least one passenger.
- Enhanced Hearing - Banshee can hear his own sonar even if it is outside the audible range of a normal human. He is so able to detect sound at any frequency or hertz so long as the vibration reach his ears. His ear drums are not vulnerable to loud noises.
- Magneto - Former Mentor, Former Teammate, Former Enemy, and Friend
- The Man in Black † - Former Ally
- Moira MacTaggert - Former Mentor, Former Teammate and Friend
- Professor X † - Former Mentor, Former Teammate, and Friend
- Beast - Former Teammate and Friend
- Darwin † - Former Teammate and Friend
- Havok † - Former Teammate and Friend
- Mystique - Former Teammate and Friend
- In the comics, Banshee is from Ireland, in the film Cassidy is American.
- Irish actor Robert Sheehan was originally cast as the character but dropped out due to commitments to Misfits.
- A ginger mutant with a gag on is seen in X-Men Origins: Wolverine, it was assumed that this mutant was Banshee. However, Banshee is said to have died before the events of X-Men Origins: Wolverine, leaving his Fate Unknown
- However, the X-Men film franchise is notoriously known for its lack of continuity and numerous inconsistencies.
- In the comics, Banshee is the father of Siryn, but in film continuity he would have been dead decades before her birth, making it impossible for them to be father and daughter.