Barsad is a turncoat, English con-man, and spy and partner of Roger Cly. In the pay of the Marquis St. Evremonde, he initially frames the Marquis' nephew, Charles Darnay by planting evidence on him on a voyage across the English Channel to England. Roger Cly, also revealed to be a spy, fakes his death and flees England, but Jerry Cruncher discovers the fraud when he attempts to 'resurrect' Cly's body for the purpose of finding valuables. Cruncher initially blames his wife's praying and thinks Roger Cly has been removed through spiritual means. When Carton discovers Barsad in Paris much later (and revealed to be Miss Pross' long-lost brother Solomon), he enlists Barsad's aid by blackmailing him to get into the prison to rescue Charles Darnay by changing places with him leading to Carton's eventual execution by guillotine. Barsad then brings Darnay out of the prison and back to his family. He is genuinely dumbfounded by Carton’s sacrifice - though not enough to change his own ways. In the final prophetic chapter, Barsad, along with many others, is revealed to die by the guillotine sometime in the future.
Barsad is described in Book 2, Chapter 3 of A Tale of Two Cities as "one of the greatest scoundrels upon the earth since accursed Judas-which he certainly did look rather like." This is a direct reference to Judas Iscariot, the man who betrayed Jesus Christ in the Bible, and is explaining that Barsad is a very untrustworthy man.