Jackson Lamb is the slovenly, chain-smoking, and sharp-minded head of Slough House. He masks his deep intelligence and Cold War-era tradecraft behind an abrasive, unhygienic exterior and a constant barrage of insults directed at his team. Despite his verbal abuse, he operates by a pragmatic, ruthless code and is fiercely protective of his marginalized agents when they face threats from the broader intelligence community.

Diana Taverner is the ambitious and morally flexible First Desk of MI5. She views intelligence work through a highly bureaucratic lens, treating operatives as disposable assets to be managed, erased, or sacrificed for political leverage. Her desire to execute off-the-books operations leads her into precarious alliances with wealthy private backers, putting her at odds with Jackson Lamb.

Peter Judd is a disgraced former politician turned PR operator who orchestrates backroom deals between wealthy financiers and MI5. He adopts a bumbling, populist persona to disarm opponents, though he privately harbors deep contempt for the public movements he manipulates. He views national security and political sentiment entirely as transactional tools to regain his own influence.

River Cartwright is a demoted MI5 agent exiled to Slough House following a career-ending blunder. He struggles with the recent death of his grandfather, a legendary Cold War spymaster, and feels the weight of failing to live up to his family's prestigious intelligence legacy. While highly trained and capable, his impulsiveness often overrides his judgment as he attempts to protect those he cares about.

Sidonie "Sid" Baker is a former MI5 agent who survived a severe head wound that previously left her presumed dead. She has spent years in Service-run recovery facilities but flees when she suspects assassins have discovered her location. Still coping with trauma and memory fragmentation, she turns to the exiled slow horses for safety, relying on her dormant operative instincts and artifacts from the past to defend herself.

Damien Cantor is a wealthy, narcissistic new-media mogul who treats national security as a commodity to trade for personal influence. He uses his financial backing of off-the-books MI5 operations to demand insider status, displaying a reckless arrogance regarding the consequences of leaking state secrets to hostile contacts.

Louisa Guy is a cynical and capable slow horse who channels her professional disillusionment into obsessive running. Still mourning the loss of her former partner, Min Harper, she maintains a detached exterior but provides a reliable, grounded presence for her colleagues. She serves as a pragmatic sounding board for River Cartwright when crises arise.

Catherine Standish is the office administrator at Slough House and a recovering alcoholic who functions as the group's moral center. She pieces together the administrative discrepancies surrounding the slow horses' deleted files and maintains order amid the office's chaos. She frequently acts as a buffer between Jackson Lamb's abrasive management style and the rest of the team.

Lech Wicinski is a technically proficient but socially isolated slow horse whose face bears extensive self-inflicted scarring to cover a slur carved into him by an extortionist. He is deeply mistrustful of his colleagues and carries intense resentment over the false accusations that derailed his career. He relies on cold calculation when confronted with danger on the streets of London.

Shirley Dander is a volatile, combative agent with a history of substance abuse and a tendency to attract physical altercations. She isolates herself due to a lingering fear that she is a jinx to her partners, adopting an aggressive swagger to ward off pity. When threatened, she eagerly channels her instability into violent retaliation.

Roderick Ho is Slough House's resident technology expert, a brilliant hacker with deep delusions regarding his own charm and physical prowess. He refers to himself in the third person and interprets every social humiliation as evidence of his superiority. Despite his painful social awkwardness, his ability to navigate digital infrastructure makes him indispensable to the team.

Desmond "Flinty" Flint is the charismatic, oafish leader of the populist Yellow Vest street movement. A former soldier, he commands genuine devotion from disaffected citizens and sincerely believes in fighting for the working class, leaving him largely oblivious to the fact that cynical political elites are orchestrating his public victories for their own gain.

Jim is a professional GRU assassin operating in the United Kingdom. He conducts his violent assignments with a detached, chillingly polite demeanor, treating state-sanctioned violence as a routine administrative task rather than an ideological crusade.

Jane is a methodical GRU assassin dispatched to eliminate the disgraced agents of Slough House. Working seamlessly alongside her partner Jim, she uses courteous misdirection and domestic disguises to gain access to her targets before attempting to execute them with bureaucratic ruthlessness.

Reece Nesmith III is an articulate, grieving American whose investigative journalist partner was murdered in Russia. Driven by a need to understand the circumstances of his partner's death, he diligently tracks down Jackson Lamb to pass on intelligence about a Russian retaliation plot, proving remarkably resilient against Lamb's abrasive bullying.

David Cartwright, known affectionately as the O.B. (Old Boy), was a legendary, high-ranking Cold War spymaster. Though recently deceased, his legacy and the secrets hidden in his library cast a long shadow over his grandson River, while his collection of espionage artifacts provides vital tools for the current generation.

Struan Loy is a destitute former slow horse living in a shipping container. He clings to the deluded hope that the intelligence service will eventually welcome him back, making him tragically susceptible to the polite deceptions of the assassins hunting him.

Tommo Doyle is a former employee of media mogul Damien Cantor who secures a position inside MI5's internal security unit. He utilizes cruel insults as misdirection to gain unauthorized access to the Service's paper archives and photograph classified personnel rosters.

Molly Doran is the cantankerous archivist for MI5 who oversees the paper records at Regent's Park. A double amputee, she is fiercely protective of her domain and astute enough to recognize when operatives like Tommo Doyle use personal insults to mask their illicit activities in her archives.

Jennifer Knox is the diligent neighbor of River Cartwright's late grandfather in Kent. She inadvertently aids the assassins hunting Sid Baker when they manipulate her into believing they are medical professionals retrieving an escaped patient.