The Outcast (also known as Nathaniel Hartford) is a former member of the Ekaterina branch, and the main antagonist of the Doublecross series. He is trying to gain leadership of the entire Cahill family. He thinks that the leaders of the Cahill family (Ian Kabra and Cara Pierce) are too young, so he sets up a competition where the young Cahills have to stop a series of four disasters from happening.
The Outcast was banned from the Ekat branch many years ago, so he is trying to get his revenge. He recruits Aleksander Spasky (Lucian), Magnus Hansen (Tomas), Melinda Toth (Lucian), Toby Griffon (Janus), Vikram Kabra (Lucian) and Patricia Oh (Ekaterina).
He infiltrates a Cahill family meeting and stages a mutiny against Ian Kabra and Cara Pierce. He gives the young Cahills a riddle to solve, and that riddle eventually leads the young Cahills to correctly believe that the Outcast is trying to recreate the Titanic disaster.
He travels to Florida where he murders Beatrice Cahill. He also gives the Cahills a riddle to solve regarding the Hindenburg air disaster in 1937. Nellie Gomez and Sammy Mourad later find information that proves the Outcast to be Grace Cahill's husband, Nathaniel Hartford.
He gets fed up with Saladin and orders Mr. Berman to get rid of the cat. The Cahills narrowly save him from death at a humane society. He detonates a nuclear bomb off of the coast of the Netherlands in an attempt to create a tsunami and flood the country. as this is happening, the Outcast views the disaster from a helicopter with Magnus Hansen and the pilot. However, the Cahills close the Dutch gates and save the country.
He tries to initiate a full scale nuclear meltdown and also the elimination of all other Cahills by spreading bees whose sting will only kill Cahills across the world but the Cahills prevent both threats It is discovered that he caused the Chernobyl disaster to happen.
In the end of Mission Atomic, the killer bees Nathaniel develops to wipe out the Cahills cover him up and sting him. As he is a Cahill, he dies.