Played by: Josh Hartnett
- An Offer You Can't Refuse: Basically why Slevin is caught doing the dirty work of two rival mobsters. It's either do as he's told or be killed. Actually, that was All According to Plan.
- Best Served Cold: "Fuck you both."
- Catch-22 Dilemma: Working for both bosses presents the problem of having the other side likely killing him for succeeding. Subverted in that Slevin is revealed to be playing them both against each other in order to avenge his parents.
- Crouching Moron, Hidden Badass: Despite his smartass behaviour and slightly dorky attitude, it turns out that Slevin was trained by a world-class assassin, and is quite The Chessmaster.
- The Chessmaster: Surprisingly, he plays both The Rabbi and The Boss like a fiddle.
- Deadpan Snarker: In part due to his condition, Slevin has no problem snarking even in the face of imminent danger.
- The Everyman: Unlike most of the other characters, Slevin is relatively down-to-earth and just normal. Subverted as it's exactly what he wants others to see him as.
- Flirting Under Fire: Slevin and Lindsey build up most of their romance in this manner. Despite the fact that Slevin is on the hit list of two warring gangs, he and Lindsey find time to flirt, go to dinner (where Slevin is able to shadow a man he has been told to kill), and spend a night together.
- Laser-Guided Tykebomb: Raised from childhood by Goodkat to one day enact a brutal revenge on those who wronged him.
- Meaningful Name: Slevin is the name of the horse race—Lucky Number Slevin. Kelevra means "bad dog", referencing his relationship to Goodkat, as well as Brikowski.
- Mr. Fanservice: He spends most of the first half of the film in just a Modesty Towel. Lindsey keeps trying to get a look.
- Obfuscating Stupidity: It turns out that his condition may or may not be faked. Behind the dorky, smartass persona is a Crouching Moron, Hidden Badass that is capable of playing two mob bosses against each other.
- Pre-Mortem One-Liner:"You destroyed everything I ever loved. Fuck you both."
- Revenge: His real motivation.
- Revenge by Proxy: Murders both The Rabbi and Boss's sons for this exact reason. Their deaths are simply a means to an end in the long run.
- The Scapegoat: Both the Rabbi and the Boss are quite happy to use him as one to get to the other. Slevin is not only fully aware of this, he's counting on it.
- Shirtless Scene: Spends a good portion of screen time in nothing but a towel.
- Trauma Conga Line: He informs Lindsey about how he came to be in his current predicament. He lost his job, comes home to find his girlfriend cheating on him and then gets mugged. Then a bunch of gangsters show up, mistake him for someone who owes them a lot of money and it gets worse from there. It turns out the whole "bad day" story was a fabrication and he deliberately engineered the case of mistaken identity as well.
- Walking Spoiler: If you read anything tagged as spoiler you might have spoiled The Reveal.
- You Killed My Father: The reason that he plays both The Boss and The Rabbi against each other, since they killed his family.
Played by: Bruce Willis
- Ask a Stupid Question...: "How'd you find out?"Goodkat: "I'm a world-class assassin, fuckhead. How'd you think I found out?"
- Badass: Effortlessly dispatches two armed guards when they try to kill him.
- Badass Moustache: Seen in flashbacks from when he was younger.
- The Chessmaster: Being a world-class assassin, he has to plan everything meticuously. Slevin got it from him.
- Guns Akimbo: When he takes out Yitzchok's bodyguards.
- Hitman with a Heart: It turns out that when Slevin was a child, he refused to kill him because he Wouldn't Hurt a Child. After realising that Slevin helped Lindsey fake her death, he lets her live and leave with him.
- Kansas City Shuffle: He discusses this trope with Nick before giving him a lethal demonstration when he breaks his neck.
- Kick the Son of a Bitch: Nick Fisher initially appears to be relatively normal, but he turns out to have been a child rapist. Not too hard to feel sorry for him after Goodkat offs him.
- Mentor Archetype: To Slevin.
- Morality Pet: Turns out to be Slevin.
- Parental Substitute: To Slevin, given his father dies in the origin.
- Professional Killer: He works as an assassin.
- Softspoken Sadist: He may be Hitman with a Heart, but he is still ruthless enough to take the jobs no one else does. And not once does he raise his voice in the entire film.
- Spanner in the Works: Had The Boss and The Rabbi offed young Slevin themselves, Goodkat wouldn't have spared Slevin and helped him with his revenge years later.
- Wouldn't Hurt a Child: After being brought in specifically to off one, he spares young Slevin and raises him as a Laser-Guided Tykebomb.
Played by: Lucy Liu
- Accidental Pervert: Though not so much on her attempt to recreate the event a second time. According to some sources, Lucy Liu actually did get a good look at Josh Hartnett during that scene...
- Flirting Under Fire: With Slevin.
- Faking the Dead: Though it turns out Goodkat never bought it.
- Hospital Hottie: She works as a coroner.
- Manic Pixie Dream Girl: Subverted in that Slevin turns out not to be The Every Man who needs a "kick" in his boring life after all.
- She Knows Too Much: The reason Goodkat feels she needs to be dealt with permanently. She survives, and when he finds out he's content to let her live.
Played by: Morgan Freeman
- Appropriated Appellation: It's actually the reason he and the rabbi stopped working together. You can't have two guys called The Boss, and he was the one everyone assumed to be the boss.
- Asshole Victim: He definitely had his fate coming.
- The Chessmaster: Prides himself of being this. He's wrong.
- Deadpan Snarker: His reaction to Slevin asking about Slim is showing his corpse and wondering why he isn't answering.
- Even Evil Has Loved Ones: His son and wife.
- Faux Affably Evil: He sounds pretty calm and restrained, but he is always making coy, veiled threats with his words.
- The Killer Becomes the Killed: The Boss and The Rabbi murder Max and most of his family. Several years later, Max's son destroys everything he cared for and kills him (plus the Rabbi) the exact same way that he did.
- Laser-Guided Karma: Slevin kills his son and then suffocates him exactly the same way The Boss did to his father.
- Meaningful Name: Why do they call him "The Boss"? Because he's the boss.
- Only Known by Their Nickname: It's eventually revealed to be Anthony in his final conversation with Shlomo (The Rabbi).
- Revenge: The plot is kicked off due him wanting to get his upon the Rabbi.
- Smug Snake: He thinks he is "the cat" and Slevin is "the mouse", but when all is said and done, he is no match for Slevin.
- Unwitting Pawn: Plays right into Slevin and Goodkat's scheme.
- Villainous Breakdown: As he is about to die, the Boss becomes erratic and screams a lot.
Played by: Ben Kingsley
- Affably Evil: He actually sounds pretty friendly when he talks to Slevin, threats to his life apart.
- Asshole Victim: While he comes off slightly more sympathetically than The Boss, he was still a murderous crime lord that killed almost an entire family for petty reasons.
- Bald of Evil: He has no hair, and fits the "evil" part pretty well.
- Even Evil Has Loved Ones: He loves his son.
- Face Death with Dignity: While the Boss gasps and despairs as he dies, The Rabbi merely gives a resigned stare to the horizon.
- The Killer Becomes the Killed: The Boss and The Rabbi murder Max and most of his family. Several years later, Max's son destroys everything they cared for and kills them the exact same way that they killed Max.
- Laser-Guided Karma: Slevin kills his son and then suffocates him exactly the same way The Rabbi did to his father.
- Meaningful Name: Why do they call him "The Rabbi"? Because he's a rabbi.
- No Honor Among Thieves: Tried to murder his former criminal partner the Boss. Why? Only one person can be the boss. Since Anthony was called The Boss most assumed he was the one ultimately calling the shots. That didn't sit well with The Rabbi. So he felt he had to remove the "confusion" over who was running the show.
- Only Known by Their Nickname: It's eventually revealed to be Shlomo in his last conversation with Anthony (The Boss).
- Religious Bruiser: A scene is dedicated to him elaborating on this concept.
- Rules Lawyer: He takes on a pretty liberal interpretation of rabbi culture to justify his gangster life.
- Sawed-Off Shotgun: The Rabbi's weapon of choice.
- Tranquil Fury: When he threatens to kill the Boss, he says it very calmly.
- Unwitting Pawn: Like his rival the Boss.
- Wicked Cultured: He discusses films with Slevin.
Yitzchok The Fairy
- Cadre of Foreign Bodyguards: The Rabbi hires two Badass Israeli bodyguards to protect his son from assassination.
- Meaningful Name: Why do they call him "The Fairy?" Because he's homosexual.
- Revenge by Proxy: The Boss wants him dead in order to hurt The Rabbi and avenge his son. Though it turns out this is actually Slevin's real reason for killing him too.
- Straight Gay: Despite the name, he's not Camp Gay.
Detective Bad Dog Brikowski
Played by: Stanley Tucci
- Asshole Victim: Slevin offs him by shooting him in the head. He murdered Slevin's mother, so it's karmic.
- Danger Takes a Backseat: How he meets his end.
- Dark Secret: He murdered Slevin's mother.
- Dirty Cop: He's a gambling addict, and it turns out he murdered Slevin's mother.
- The Gambling Addict: The Boss's bookie used to be his bookie. Brikowski even says he basically handed over his paychecks to him.
- Jerkass: Not a very likable guy when first introduced and by the end is even less sympathetic.
- Laser-Guided Karma: Invoked by Slevin, who makes sure he knows exactly why he's about to die before killing him.
- Trapped by Gambling Debts: Its implied that it was so bad he killed Slevin's mother to wipe his debts clean.