Jackson Lamb is the slovenly, flatulent, and abrasive chief of Slough House, MI5's dumping ground for disgraced agents. A former Cold War field agent who survived dangerous operations, he currently spends his days drinking, sleeping, and relentlessly insulting his subordinates. Beneath his repulsive exterior and apparent apathy lies a sharp intelligence and a strict personal moral code regarding the treatment of agents.

River is a capable and ambitious young MI5 agent exiled to Slough House after a disastrous training simulation. Raised by his legendary spy grandfather, River trained for a career in intelligence since childhood and deeply resents his current administrative banishment. Eager to see real action and convinced he was set up to fail, he takes unauthorized risks to investigate an unfolding hostage situation.

Catherine is the quiet, rule-abiding office manager at Slough House and a recovering alcoholic. She still mourns the death of her former boss, Charles Partner, believing she failed to prevent his suicide. Dismissed by many as a forgettable administrator living in the past, she possesses sharp observational skills, steady nerves, and an unwavering sense of duty.

Diana Taverner is the ruthlessly ambitious Second Desk at MI5, currently acting head of the Service while her superior is away. Operating out of the gleaming Regent's Park headquarters, she embodies the modern, highly corporate intelligence world. She focuses heavily on public relations, political maneuvering, and advancing her own career through high-stakes operations.

Sid is a highly competent MI5 agent stationed at Slough House. Unlike the other "slow horses," her exact reason for banishment remains mysterious, and she executes field tasks with precision. Motivated to join the Service following the 2005 London bombings, she is sharp, proactive, and closely monitors River's unauthorized investigations.

Spider is an ambitious intelligence agent working at MI5's Regent's Park headquarters. Dark-haired and socially adept, he operates strictly by the rules of self-protection and career advancement. He enjoys his position of power under Taverner and actively works to distance himself from his former colleagues who fail to match his upward trajectory.

Hassan is a 19-year-old British Pakistani university student and aspiring stand-up comedian. Abducted off the street by a white nationalist group, he realizes his captors chose him purely based on his skin color. Despite his intense terror and the threat of decapitation, he relies on his comedic internal monologue to cope with captivity and maintain his dignity.

Hobden is a disgraced former journalist who lost his career and mainstream connections after his ties to a right-wing extremist political party were exposed. He maintains a fringe network and continues to investigate political conspiracies, hoping to leverage stolen information to regain his lost status and influence.

Known within the Service as "the O.B." (Old Bastard), David is River's grandfather and a legendary retired MI5 agent from the Cold War era. He raised River from the age of nine and steeped him in the traditions of espionage. He serves as River's sounding board, offering historical context, cautious advice, and a link to a bygone era of spying.

Min is an anxious, forgetful MI5 agent relegated to Slough House after accidentally leaving a classified CD on a train. The professional mistake derailed his personal life, leading to a separation from his wife. He is gentle but deeply frustrated by his stagnant career, finding solace in a growing connection with a fellow slow horse.

Louisa is a capable but disillusioned slow horse sent to Slough House after losing a target who later flooded the black market with weapons. Frustrated with her tedious administrative duties, she harbors a quick temper and occasionally drinks to cope with her career stagnation. She easily snaps back into a highly competent operative when danger arises.

Roderick is a brilliant but intensely anti-social computer and hacking expert at Slough House. Arrogant and lacking in basic empathy, he lives alone, plays loud music at night, and actively despises most of his coworkers. Despite his off-putting personality, his ability to manipulate digital infrastructure proves vital during unauthorized investigations.

Jed is a former member of "the Dogs" (MI5's internal security team) who has been banished to Slough House. Resentful of his demotion and Lamb's management, he actively seeks to ingratiate himself with Regent's Park by secretly planting recording devices in his own office to gather leverage.

Duffy is the head of "the Dogs," MI5's internal security and tactical enforcement branch. Loyal entirely to Regent's Park management, he is a ruthless, unquestioning enforcer who handles the physical detention and operational cleanup required to maintain the Service's secrets.

Peter Judd (often referred to as PJ) is a charismatic and highly ambitious Conservative politician with his eyes set on becoming Prime Minister. He publicly projects a polished establishment image while secretly harboring extreme right-wing views, making him an ideal target for political blackmail.

Curly is a violent, radicalized member of the right-wing extremist group "Voice of Albion." Deeply committed to his racist, white-supremacist ideology, he enthusiastically plans to execute their hostage on camera to spark a nationalist uprising, viewing himself as a patriot rather than a criminal.

Larry is a member of the "Voice of Albion" terrorist cell holding Hassan hostage. Unlike the zealous Curly, Larry is hesitant about the extreme violence of their plot. As the reality of their situation sets in, he becomes increasingly fearful of the legal and physical consequences of their actions.

Alan Black (also operating under the alias Dermot Radcliffe) is a former slow horse acting as an embedded undercover agent within the Voice of Albion. Operating under the name "Moe," he secretly reports to MI5 while posing as a right-wing extremist to manipulate the group's actions from the inside.

Struan is an agent at Slough House known for his attempts at humor in an otherwise bleak environment. When pressure mounts from Regent's Park, his loyalty to his fellow slow horses is tested by Taverner, who offers him false promises of career rehabilitation.

Kay is one of the marginalized agents working at Slough House. She passes her time in administrative obscurity until the entire team is drawn into the fallout of the hostage crisis and targeted by internal security.

Charles was a high-ranking Cold War-era MI5 agent and Catherine Standish's former boss. His apparent suicide years ago remains a heavy emotional burden on Catherine, though the true circumstances of his death are tightly guarded secrets known only to top-tier operatives.