BIR Agent Eric Sebso
Van Alden's partner.
- And Call Him "George"!: He dies when Van Alden tries to convert him to Christianity and/or make him confess that he ruined the case against Jimmy on purpose.
- Bumbling Sidekick: As part of Obfuscating Stupidity, Sebso plays the part of the incompetent stooge. He mishandles evidence, he's slow-witted and is a hindrance to Van Alden.
- Character Death: He's accidentally killed by a crazed Van Alden during a forced baptism.
- Dirty Cop: He's a prohibition agent, but he secretly works to sabotage his own investigation at the behest of Nucky Thompson.
- Faux Affably Evil: Sebso is disarmingly clumsy and foolish, or at least that's the part he plays. In reality, he's a ruthless Dirty Cop.
- Informed Judaism: At first he seems completely secular and the only mention of his heritage is him speaking Yiddish as part of a joke in "Broadway Limited". In "Paris Green", however, he adamantly refuses Van Alden's attempt to forcibly convert him and which leads to his death.
- Last-Name Basis: His first name isn't revealed until after his death.
- The Mole: For Nucky Thompson and his syndicate.
- Obfuscating Stupidity: Initially, Sebso just seems wildly incompetent. It's later revealed that he's actually corrupt, and his supposed incompetence is due to him actively working against the investigation he's part of.
- Pre-emptive Declaration: He uses this before killing a witness he was paid to silence.
- Wounded Gazelle Gambit: After killing said witness, he hits himself in the head with a rock to make it look like he was attacked.
- Yiddish as a Second Language: As part of his Informed Judaism.
BIR Agent Warren Knox/BoI Agent James Tolliver
- Ambiguous Disorder: He comes off as having an odd, slightly off kilter demeanor and way of acting. While much of his strange behavior is later revealed to be an act, it is nonetheless apparent that there is something mentally wrong with him given his single-minded obsession with gaining recognition for enforcing the letter of the law at all costs. Additionally, unlike the perpetually grim Van Alden, he has a bit of a sadistic streak as evidenced by the gratification he derives from breaking down Eddie Kessler during an interrogation as well as his smug amusement over Eli's helplessness while using his son as leverage to coerce him into incriminating Nucky.Knox: "Do you think I'm crazy?"Eli:"I don't know what you are..."
- Asshole Victim: No one shed any tears when Eli bashed his brains in
- Ax-Crazy: Increasingly revealed to be this as Season 4 progresses.
- Been There, Shaped History: He's on first name basis with J. Edgar Hoover.
- Big Bad: In Season 4 along with Dr. Narcisse as part of the story arc's Big-Bad Ensemble. His investigation into the existence of a nationwide criminal conspiracy on behalf of the BOI poses the most direct threat to Nucky Thompson and ultimately succeeds in placing his entire organization within Hoover's crosshairs by the end of the season (albeit not quite the way he intended).
- Bitchin Sheeps Clothing: His All-American good lucks serve as a convenient mask for his ruthlessness and sadism.
- Bumbling Sidekick / Clueless Deputy: Introduced as one in "New York Sour", but it's actually an act.
- Cassandra Truth: He's (correctly) convinced that the major American crime bosses in New York, Atlantic City, Chicago, and Florida are all working together. However, his superiors don't believe him, no matter how much evidence he gathers.
- Cowboy Cop
- Dude, Where's My Respect?: He's unamused by Hoover stealing his theories and not giving priority to his investigations.
- Gone Horribly Right: As per his page quote his plan to break the weak link in Thompson's chain proves so effective that Eddie Kessler winds up killing himself after doing exactly what Knox wanted and informing on Nucky.
- Gosh Darn It to Heck!: He uses these to avoid swearing.
- Gratuitous German: As a former member of Military Intelligence during World War I, he speaks it fluently and drops some sentences in "Erlkonig".
- Fair Cop
- Faux Affably Evil: He(Almost) never raises his voice, and gets along very well with Eli's family, this is all an act, of course. He also attempts to induce Stockholm Syndrome on Eddie when he's interrogating him and on Eli when he becomes his informant.
- FBI Agent: He is an undercover agent sent by J. Edgar Hoover to root out corruption in the BIR and take down bootleggers.
- Foil: To Sebso. Both have their own goals and hide them from their superiors by playing Obfuscating Stupidity, but while Sebso is a corrupt agent on Nucky's pay roll that is sabotaging Van Alden's work, Knox (AKA Tolliver) is an undercover BoI agent that really wants to get rid of alcohol and corruption within the BIR, starting by the dirty agents Sawicki and Elliot.
- Indy Ploy: After being rebuffed by the bank holding Eddie's account(s) (holding in turn Nucky's money), Eli trusts him to retrieve it. Knox then tells the banker that Eddie is suspicious of being an Anarchist activist and convinces him to turn in the contents after threatening with prosecution for obstruction of justice.
- Jerkass: He pushes the Relative Button a little too much with Eli, and his interrogation of Eddie Kessler, which includes but is not limited to bringing up his past, which includes adultery, embezzlement, and the threat of deportation if he doesn't cooperate, causes him to Commit Suicide
- Knight Templar: Despite posing as a corrupt BIR cop on the take, he is subsequently revealed to be an idealistic federal agent unwaveringly committed to purging the U.S. of crime. However, the sheer brutality of his methods along with the sadism he exhibits when employing them make him a harbinger for the authoritarianism his organization will soon come to embody under Hoover's leadership.
- Manipulative Bastard: Early in the season 4 premier he sees Agent Sawicki take a bribe and also hears a bootlegger complaining about how the bootlegger's warehouse has been broken into so many times that he's rigged a lethal booby trap for the next thief. Tolliver promptly pretends to receive a tip that sends Sawicki and himself to the bootlegger's warehouse, and pretends to be so incompetent that Sawicki actually orders him to stay by the car... and out of harm's way. Sawicki gets mortally wounded, Tolliver kills the bootlegger as he comes to see who tripped his trap, and then calmly tells the dying Sawicki that he'll call for help... as soon as he gets over the shock that is affecting him. He then simply watches Sawicki die.
- New Meat: So it appears.
- Obfuscating Stupidity: After spending the season premiere looking like a laughably gullible guy, he shows his true colors as the second coming of Van Alden.
- Rabid Cop
- Rasputinian Death: He is punched, kicked, choked and finally bludgeoned to death by Eli.
- Sarcastic Confession: After he becomes the head of the Prohibition enforcement in Atlantic City and meets Nucky, he tells him: "I do intend to take my duties with the utmost seriousness and to enforce the existing laws to the best of my abilities." Everybody is shocked before he adds that he was just joking. Actually, Tolliver was serious since he's an undercover BoI agent.
- Smug Snake
- Softspoken Sadist
- Suspiciously Similar Substitute: To Van Alden.
- The Teetotaler
- Undercover Cop Reveal: He is actually an undercover Bureau of Investigation agent posing as a corrupt BIR agent.
- Villainous Breakdown: After getting ignored one to many times by his superiors, and having an important sting operation go awry under his watch, what little sanity Knox possesses begins to quickly fray.
- Walking Spoiler
BIR Agent Stanley "Stan" Sawicki
- Cruel and Unusual Death: Shot by a shotgun tied to a door, left to choke on his blood while his "partner" stares at him joking he'll do nothing.
- Dirty Cop: In Season 3 he can be considered a member of Nucky's organization.
- The Eeyore: In Season 2, in contrast to Clarkson. He does not believe that they can Clean Up the Town and as the season goes on he loses all faith in the job.
- Karmic Death: If he did in fact set Owen up, his own death can be practically considered Laser-Guided Karma. Fed up with Sawicki's corruption, his new partner Knox talks him into making a bust knowing that it will get Sawicki killed.
- Red Oni, Blue Oni: Blue to Clarkson's red.
- Those Two Guys: With Clarkson in Season 2.
- Uncertain Doom: As of the end of Season 3. He's back for the first ep of Season 4. Then dies.
- What Happened to the Mouse?: He goes with Owen to kill Masseria in "A Man, A Plan...". Owen's body is returned in a box, but we didn't hear from Sawicki for the rest of Season 3. It's implied that he may have set Owen up.
- You Are in Command Now: After Clarkson is WIA and Van Alden runs away, leaving him as the only "prohi" in town.
BIR Agent Clarkson
- Accidental Pervert: He gives a guide ("If Jesus ever came to Atlantic City") to Van Alden's wife Rose, believing that it lists the town's churches. It's actually a guide about the places where Jesus would not go.
- Clean Up the Town: And the Squad too. He is a firm believer of this.
- Cowboy Cop: He convinces Sawicki to overrule Van Alden's orders and investigate Mickey Doyle - and perhaps Van Alden himself.
- No Good Deed Goes Unpunished: He gets an explosion to the face for his trouble.
- Last-Name Basis
- Red Oni, Blue Oni: Red to Sawicki's blue.
- Those Two Guys: With Sawicki in Season 2.
- Two-Faced: While he agonizes after catching an explosion to one side.
- What Happened to the Mouse?: Word of God says that he didn't survive his injuries.
Mike Malone a.k.a. Mike D'Angelo
One of Eliot Ness' agents, acting as a mole in Al Capone's organization where he has become a well respected and high-ranking member.
- Fake Nationality: In-universe, he is an Irish-American agent posing as an Italian-American gangster. The actor is actually Italian-American.
- Historical-Domain Character: He really existed.
- The Infiltration / Reverse Mole: A non-corrupt fed in the Chicago Outfit.
- The Plan: After learning that Van Alden is a former BIR agent being wanted for murder, he pressures him and Eli into stealing the ledgers detailing Capone's tax evasions, ensuring that he will be judged in federal court by a jury he can't buy.
- Remember the New Guy?: Because of the unplanned 7 years-long time jump between seasons 4 and 5.
- Undercover Cop Reveal: He does this to Van Alden when he addresses him as "Nelson", then repeats it by calling Eli "Sheriff Thompson".
A Chicago Treasury agent who has publicly declared that he will bring down Al Capone.
- Brainy Brunette
- Crusading Lawyer: Not even her superiors are interested in persecuting bootleggers.
- Deadpan Snarker
- Emotionless Girl
- Enemy Mine: Nucky makes her an offer she can't refuse in season 3 - team up with him to take down Harry Daugherty and George Remus. Nucky gets his enemies taken care of, and Esther rebuilds her career from the damage Nucky's failed trial did to it.
- Finishing Each Other's Sentences: Randolph and her assistant use this trope to great effect as an interrogation technique when trying to needle Ray Halloran for information. Halloran even makes a joke about the two of them being married (they're actually only sleeping with each other).
- Go-Karting with Bowser: In Season 3 she accepts Nucky's invitation to breakfast.
- Good Is Not Nice: However, she's one of the few law enforcement characters who isn't corrupt or a Knight Templar.
- Hello, Attorney!
- Heroine Antagonist: She's probably the most moral character on the show.
- Jurisdiction Friction: Has a brief case of it when she moves in and takes over a part of Nelson's office, including his desk.
- No Celebrities Were Harmed: Replaces the real Assistant Attorney General during the Prohibition era, Mabel Walker Willebrandt.
- Reassigned to Antarctica: After her big case against Nucky flounders, she is assigned to prosecute bit-time bootleggers on night court.
- Silk Hiding Steel
- Smart People Know Latin
- The Smurfette Principle: The only female employee in the building, this being the 1920s and everything.
- Stealth Insult: Pulls a few on Halloran.
- Sympathetic Inspector Antagonist: She's a much better person than Nucky. Unlike other law enforcement characters, she's neither cruel nor corrupt. Before she was assigned to prosecute Nucky, she worked as a lawyer representing the most disadvantaged: draft dodgers and prostitutes.
- Unresolved Sexual Tension: With Nucky in Season 3.
BoI Director J. Edgar Hoover
A rising federal star who serves as Acting Director of the Bureau of Investigation.
- Aborted Arc: At the end of S4, he forces Dr. Narcisse to become a snitch for him and bring down the UNIA from the inside. This is not revisited in S5, which is set 7 years later and Hoover's character is absent.
- Always Gets His Man: Introduced as an incorruptible law enforcer who is determined to scourge corruption from both sides of the law.
- Fair Cop/Young and in Charge: He is only 29 when he becomes acting director of the BoI. Frederick Elliot doesn't take Hoover seriously because of his age and calls him a child upon being questioned by him when they first meet.
- FBI Agent: A proto-agent, as the Bureau of Investigation would become the FBI in 1935.
- First-Name Basis: He refers to a trusted agent as Jim and has no problem with the agent calling him Edgar in return. This is subverted as their relationship goes sour when he demands to be addressed as "Director Hoover."
- Foil: An untainted official, unlike the crooked agents of the BIR.
- Glad I Thought of It: After dismissing it, he steals Tolliver's theory about the existence of a nation-wide criminal network.
- Greater-Scope Villain: In Season 4. As the new Director of the Bureau of Investigation, he institutes highly selective hiring standards and ruthlessly efficient investigation procedures that provide his organization with the resources to infiltrate and thoroughly investigate Nucky's massive criminal empire. It is this fact that enables BoI agents such as Jim Tolliver to pose a far greater threat to the Atlantic City political machine than BRI Agent Van Alden ever did.
- Historical-Domain Character: One of the few non-fictional law enforcers.
- Historical Person Punchline: In his first episode, "Resignation".
- Inspector Javert: Is obsessed with bringing down Marcus Garvey since he believes the civil rights movement is a threat to America. As a result he largely ignores the issue of organized crime.
- It Will Never Catch On: Scoffs at the idea of a large-scale network of organized crime (which is very much Truth in Television).
- The Comically Serious: Especially when contrasted with George Remus.
- Under New Management/Jurisdiction Friction: His Bureau of Investigation takes over the corrupt BIR. Bootlegging is the BIR's jurisdiction but the BoI is brought in due to the corruption among BIR agents.
- Young Future Famous People: He's introduced at age 29, with his historical personality already fully developed.
BIR Supervising Agent Frederick "Fred" Elliot
- All There in the Manual: His name.
- The Bus Came Back: Appears again in Season 4 after being absent during 3.
- By-the-Book Cop
- Da Chief
- Demoted to Extra: In Season 2.
- Dirty Cop: Shown to be working with Nucky in season 4.
- Not so Fast, Bucko!: About half his dialogue with Van Alden.
- Obstructive Bureaucrat: To Van Alden. Elliot is only interested in the fight against illegal liquor, not caring about its use to finance organized crime.
- Oh, Crap!: His reaction when he realizes his bosses are tired of his corruption and inaction and have sent in the BoI to clean up his mess.
- Police are Useless: "Resignation" reveals that he is a Dirty Cop. And probably was one since the beginning.
- Rousing Speech: In the Pilot, before the new BIR recruits.
- Stupid Neutral: Elliot is aware of Nucky's many crimes as a corrupt politician including embezzling, tax evasion and election fraud, and his number one agent believes that he is related to five murders and another attempt. What does he do? Nothing! He is a Prohibition agent, he and his men are not going to persecute other crime but bootlegging. Even if bootlegging is rampant in the town because of a certain Mr. Thompson's influence in the first place. What, passing the evidence to the correct department, you ask? Why? He's corrupt. That's why.