Haskell Twain is a codebreaker and employee of Raymond Reddington.
Known as "the Polyglot", Twain was a codebreaker specializing in decoding redacted files. He did some work for Mossad, decrypting black budgets. By studying the redacted documents and comparing it to more official material such as executive orders and defense reports and authorizations, he was able to figure out the contents of the redacted documents. He had a gambling addiction, and blamed Reddington for triggering a relapse at a casino and costing him a lot of money.
When Samar mentions Twain as a potential help in decoding the redacted Subproject 7 files, Reddington takes Liz, who poses as a blogger, and repays Twain the money he lost gambling, with interest. Twain gives them the decoded Subproject 7 files, putting them on the trail of Linus Creel.