* Jack "The Slasher" O'Neill: A tough, experienced SAS soldier, haunted by a past that includes a brutal mission in the Falklands War. He is deeply loyal to his comrades and fiercely protective of those he considers his "brothers."
* Sergeant Danny "Mac" McArthur: A seasoned SAS officer, Mac is Jack's commanding officer and a trusted friend. He is a calm and level-headed leader, but also has a dark past that he tries to keep hidden.
* Lance Corporal "Paddy" O'Reilly: A young and enthusiastic SAS soldier who is eager to prove himself. He is energetic, brave, and has a strong moral compass.
* Corporal "Baz" Baxter: A skilled and resourceful SAS soldier, Baz is a valuable asset to the team but struggles with inner demons and a drinking problem.
These four characters form the core of the SAS team that is deployed on a dangerous mission in the Irish Republic. The book explores their individual struggles, their bonds of brotherhood, and the moral complexities of their work.
It's important to note that "Brother in Arms" is a multi-faceted story with many supporting characters who contribute to the overall narrative. However, the core relationship and the primary focus are on Jack, Mac, Paddy, and Baz.