John Barsad, real name Solomon Pross, is described as:

Age, about forty years; height, about five feet nine; black hair; complexion dark; generally, rather handsome visage; eyes dark, face thin, long, and sallow; nose aquiline, but not straight, having a peculiar inclination towards the left cheek; expression, therefore sinister.

He first appears in the third chapter of the second book when the Solicitor General examines him at Charles Darnay's trial.

Had he ever been a spy himself? No, he scorned the base insinuation... Ever been in prison? Certainly not... Ever live by cheating at play? Never. Ever live by play? Not more than other gentlemen do.

Despite his denials the reader later finds out that Barsad was

a hired spy and traitor, an unblushing trafficker in blood, and one of the greatest scoundrels upon earth since accursed Judas

He starts off spying for the English, claiming he did so out of patriotism, but switches to spying for the French as soon as it suits him. He is the person who frames Charles Darnay. At the end of the novel the author suggests he will die at the guillotine during the Reign of Terror.

John Barsad, or Solomon Pross, to give him his real name, is a spy. He's one of the prosecution witnesses in the treason trial of Charles Darnay in Book 2, Chapter 2 of A Tale of Two Cities. He claims to be a fine, upstanding patriot, motivated by nothing more than love of country. But this is a complete lie, as subsequent events will prove. For Barsad is one of life's opportunists. The turmoil generated by the French Revolution gives disreputable characters like Barsad the chance to change from a nobody into a somebody almost overnight. Far from being a patriot, Barsad is actually a traitor to whichever country he happens to inhabit at any given time. His utter amorality is on display when he agrees to become a spy for the French Revolutionary regime. Barsad no more believes in the ideals of the Revolution than he does in his own country. As always, he's simply looking out for Number One.