Arthur Ketch is first mentioned while Lady Toni Bevell is torturing Sam for information. When all their methods prove fruitless, Ms. Watt suggests bringing in Ketch; however, Toni is vehemently against it.
He is also shown in a flashback killing a vampire, identifiable by the tattoo on his hand.
Later on, Mick Davies retrieves Toni and informs her that he has sent for Mr. Ketch, who is shown preparing to leave the UK. This leaves Toni visibly uncomfortable.
In American Nightmare, Mr. Ketch drives up to the Impala on a motorcycle (first seen in the previous episode) and stares it at for a moment before driving away. He later tracks down and kills Magda Peterson, telling someone on the phone that he cleaned up the Winchesters mess and they were right about the Winchesters being unable to kill Magda.
In LOTUS, Arthur rescues the Winchesters and Castiel from the Secret Service and orders Castiel to wipe their memory before introducing himself. That night, Arthur explains that his job is to "strongly encourage" the Winchesters cooperation with the British Men of Letters and was sent by Mick Davies after Sam called him and hung up. Arthur has Castiel confirm that he's not lying and shows them his arsenal of weapons, including a hyperbolic pulse generator capable of exorcising a demon from its vessel. Recognizing its potential, Sam asks to borrow it and for Arthur to trust them. Arthur eventually gives the Winchesters the generator, enabling them to force Lucifer from Jefferson Rooney and send him back to Lucifer's Cage.
Six weeks later, Castiel contacts Arthur and Mick to help him and Mary find Sam and Dean who have been taken prisoner by the Secret Service. The two men are impressed to learn that the brothers had in fact borrowed Arthur's gadget in order to deal with Lucifer himself. They agree to help in hopes that it will earn them the trust of the American hunters, and through their connections, Arthur and Mick locate Sam and Dean near Site 94.
Once the four encounter Sam and Dean, Arthur is unimpressed to learn that they left survivors. Sam insists the people pursuing them were just soldiers doing their job, though Arthur considers it "unprofessional". Sam, Dean and Castiel head to Mary's car while glaring at Arthur suspiciously. Arthur and Mick watch them drive off and share a knowing look at each other. It is later revealed that Arthur went back and killed all those involved in Sam and Dean's imprisonment, including Agent Rick, Agent Camp, the soldiers and the coroner. Mick reports this while noting that cleaning up loose ends is Arthur's job.
Equipment
As a member of the British Men of Letters, he has access to its vast weaponry. The engineers of the British Men of Letters have spent years blending sorcery and technology to produce an advanced arsenal to combat supernatural threats.
Vampire Irradiation Device.
Vampire Irradiation Gun: The British Men of Letters don't always have to cut off the heads of vampires to eliminate them as they have developed a tool that uses radiation to reorder the DNA of a vampire which turns the vampire's own blood into poison that is lethal to them. It is said to be very effective on large vampire nests.
Hyperbolic pulse generator: A device capable to targeting specific angels or demons and exorcising them safely without harming the host. It is capable of removing even an archangel.
Personality
Arthur presents himself as a gentleman of sorts with a strong British accent. He has no qualms about killing people and considers it his job to clean up loose ends. He believes stalking the Winchesters and killing all those they choose to spare as being "professional". He can be a bit of a show off as seen during his formal introduction to Sam and Dean, when he blew up a vehicle belonging to the Secret Service and gladly presented the Winchesters with his vast weaponry. His well-dressed appearance and charming manners clearly hide a much more sinister personality, which Sam, Dean and Castiel are beginning to notice.
While Ms. Watt implies he's a very proficient torturer, Toni describes him as a psychopath.
His name is possibly a reference to the famous executioner Jack Ketch, who made a name for himself as the somewhat barbaric executioner of political offenders in the late 1600's.
Arthur describes himself as an artist who paints in many colors.
