Barney Thompson was a WWI comrade and old friend of Thomas Shelby's . He was admitted to a mental asylum shortly after the war after committing an unknown crime, presumably due to his severe shell-shock and was certified insane.
He served as a sniper in WWI, trained by the Royal Marines. He gained a reputation of being a very skilled marksman, and according to Thomas, never missed a target.
Prone to manic episodes and hearing voices, Barney was only able to be calmed down by Thomas, for whom Barney had deep respect as Thomas was his superior in the War. At some point in France, he indirectly gave Thomas a sexually transmitted disease, as the pair both slept with the same French waitress.
After being broken out of the asylum by the Peaky Blinders, Barney was key in the assassination attempt on Oswald Mosley at a rally at Bingley Hall, tasked with sniping him on stage from a lighting control room. However, he was killed with a silenced pistol at point-blank range seconds before taking the shot. His killer remains unknown.