We tend to think of John Wilkes Booth as a lone gunman, but he actually had a whole team of conspirators working with him. The only reason we don't remember them as well as we remember him is that they didn't do as good a job as he did.
The other people in Booth's conspiracy to kill off the top Federal leaders share several qualities:
In real history, there were about eight or nine other conspirators, but in this book, we focus on only four:
Mary Surratt was a widow who loved Booth and the South, so she carries a message for Booth to the other conspirators. And for that, she gets executed (13.19). (This was pretty controversial back then; and was actually the basis of a recent movie, The Conspirator.)
Lewis Powell was a young, former Confederate soldier and was, apparently, crazy dedicated to the South (13.20). Powell's job was to kill Secretary of State Seward, but since Powell didn't know the city, he was supposed to be led by David Herold, whose main quality is that he was nervous. Herold led Powell to Seward's house, where Powell shot and stabbed a bunch of people—but he failed to kill anyone and when he ran out of the house to escape, Herold had already run away (13.74-84). (In real life, Powell and Herold get hanged.)
George Atzerodt was originally recruited by Booth when the plan was to kidnap Lincoln, and wasn't comfortable with the whole "let's shoot a bunch of people" plan (13.32). His job was to shoot Vice-President Johnson, but Atzerodt was smart, and just got drunk instead (13.85). (And then, yes, he gets hanged.)