Hitman: Enemy Within is the first novel in the Hitman series. It was written by William C. Dietz and released on August 28 2007, from Del Rey Books. The novel's plot revolves around a rival murder-for-hire organization, known as Puissance Treize (French for "Power Thirteen"), attempting to destroy Agent 47's employer, the International Contract Agency (ICA). The book's tagline is: The clone assassin has been played long enough– now it’s more than a game.
- Agent 47 - The protagonist of the Hitman series; a genetically-altered clone assassin who utilizes various disguises while on-assignment.
- Diana - Agent 47's personal contact within The Agency.
- Cassandra Murphy a.k.a. Marla - A young female assassin working for Puissance Treize.
- Mr. Nu - Described as "Management" of the Agency. He is attempting to identify a traitor within the Agency and meets with Agent 47 regarding his assignments relating to this objective.
- Aristotle Thorakis - A Greek shipping magnate and a board member of The Agency.
- Pierre Douay - A Frenchman affiliated with Puissance Treize who influences Thorakis throughout the novel.
- Ali bin Ahmed bin Saleh Al-Fulani - A wealthy Moroccan businessman who hosts lavish pedophilic parties for foreigners and is also a key member of Puissance Treize.