Bestselling-but-bored playboy mystery writer who, after helping Kate Beckett and the NYPD find a killer basing his murders off his books, pulled strings to get himself attached to Beckett's unit, ostensibly to research a character based on her but really because he likes solving mysteries and is attracted to Beckett.
Action Dad: He handles himself well against thugs. See his brief partnership with Slaughter for details.
Action Survivor: The more time he spends with the police force, the better he gets at surviving trouble.
Adorkable: Castle often gets giddy for the dorkiest reasons.
Agent Mulder: Castle loves wild theories. However, unlike the actual Mulder, Castle gives the strong impression of wanting to believe these things simply because it's more fun that way.
In one case involving zombies ("Undead Again"), he outright states that he keeps insisting on the possibility of zombies just to irritate Beckett.
He's genuinely open-minded to the possibility that things like ghosts and Bigfoot actually exist, but that is also in keeping with his overall attitude of "wouldn't it be cool if they did?!" rather than a hardened conviction that they do.
Alliterative Name: His birth name is Richard (Alexander) Rogers. When he became a novelist, he changed it to Richard Edgar Castle.
Bumbling Dad: Zigzagged from "Alexis is the one raising him" to "Pillar of Support" such as in "Vampire Weekend" when Alexis is at a party with spiked punch, and her friend gets drunk, and he acts exactly how every parent in the world would hope to. When it matters he's the best dad in the world; when it doesn't, he's the best friend in the world. Which makes him the best dad in the world again.
Camp Straight: Castle was called a metrosexual in "The Third Man," though he's a pretty mild example. He has a fine appreciation for good interior decorating and fashion.
He's a lot more mature and grown-up after working with the police for a while than he was when the series began. This can especially be seen in "Tick, Tick, Tick...Boom", two episodes in which, like the pilot, someone was inspired by Castle's novels to commit murder. It has been explicitly stated that these episodes were meant to show the difference between Castle from Season 1 and Castle from Season 2. Unlike the premiere episode, "Flowers for Your Grave" in which Castle was quite flippant and arrogant about the entire situation, Castle is appropriately devastated at the idea that someone has once again died because of his books. In fact, Beckett has to talk him down from taking blame for the entire situation.
Castle starts off doing "volunteer homicide detective" work because he thinks it's cool and because he's got a shine on his "muse,” and his flippant manner shows it. By "47 Seconds," Castle is working with the police because he honestly wants to make a difference in the lives of the victims and their families, despite finding out that Beckett lied to him about being aware of his feelings for her, making him question Beckett's intentions.
Crouching Moron, Hidden Badass: Normally, he's, as Beckett describes him, like a "9 year-old on a sugar high who doesn't take anything seriously" and always has crazy ideas about why crimes happen, but when Alexis is kidnapped and the man who took her won't talk, Castle tortures him...makes him scream out in pain. When Beckett tells him afterwards that she didn't think he had that side to him, he replies "Well, when it comes to the people I love, I do."
The Dandy: He loves to dress well, as well as indulging his love of original wainscotting. He isn't #9 on the New York Ledger's Most Eligible Bachelors list for nothing - although that is a drop from being #7 the previous year. Alexis is not amused at her father having the recognition.
Deadpan Snarker: A large appeal of the show is the snarky banter between Castle and Beckett.
Disappeared Dad: Castle doesn't know who his father is. Unlike many other examples of the trope, however, he seems quite free from bitterness or Wangst over this fact, because as he says, it gives him the chance to imagine his father as whatever he wants. This is shown in "Suicide Squeeze.” In "Linchpin" (4x16), Sophia Turner claims that Castle's father was in the CIA; however, Beckett considers this as one of the many lies Sophia has told them before revealing her true allegiances. Halfway through season five we learn that Sophie was telling the truth : Castle's dad is CIA - and he is the reason why Castle met Sophie in first place.
Flat Earth Atheist: Inverted. While solving seemingly mysterious cases with mundane solutions on a somewhat regular basis he still falls for cases with a touch of the supernatural on it. As he is gleefully quick to point out, there is often an edge of Maybe Magic, Maybe Mundane to several of the cases of this nature that they've worked.
Handsome Lech: in "Cloudy With A Chance Of Murder", Castle comments on what Beckett is wearing just so he can watch her change clothes.
Green-Eyed Monster: It's clear for most of the series that he doesn't like it when other men grab Beckett's attention, and unlike her he is none too subtle about it.
Heroes Want Redheads: Played with in that quite a few of the women in Castle's lives are redheads, including his first ex-wife, his mother, and his daughter, but the succession of love interests he's had since are not. Kyra Blaine, Ellie Monroe and of course Beckett are all brunettes, while his second ex-wife Gina is a blonde.
I Love You Because I Can't Control You: Beckett is a fiercely independent woman. Rather than Castle trying to find a way to 'win' her, it's suggested that part of her appeal is in the fact that she can control him.
It's for a Book: It actually is; Nikki Heat books are available in a book store near you. Except it eventually isn't because he's truly there for his muse.
I Will Wait for You: Long after their attraction to each other is clear, he's content to let Beckett work through her issues and support her until she's ready. In "47 Seconds", in spite of him finding out that Beckett actually remembered his confession of love after she was shot, he still chooses to continue working with her. In "Always", he finally gives up when he confesses again to her to keep her off her mother's case, but to no avail. This forces Beckett, after being barely rescued from falling off a building, to finally admit her feelings for him.
Jerk with a Heart of Gold: Castle often comes off as a typically self-centered, thoughtless and narcissistic playboy, but he loves his daughter and his mother deeply, and is revealed to have Hidden Depths of caring and loyalty. While he's not shy about flirting with Beckett, it's also made abundantly clear that he genuinely cares for and respects her rather than simply viewing her as a potential conquest.
The Lancer: To Beckett; he's goofy and laid back to contrast her dangerous stoicism.
Last Name Basis: No regulars call him Richard but his mother. Although he is Rick or Richard to several ex-girlfriends, his ex-wives, and Beckett during significant moments or when she's emotionally fragile.
Man Child: Will often spend time playing with toys. Sometimes his daughter's there but often she isn't. He bought property on the moon! Last month.
Meaningful Name: If you think about it in terms of him being a Stephen King expy, which he is according to Word of God, "Richard" is the first name from Stephen King's infamous pseudonym and a Castle is where a King lives.
Minored in Asskicking: Castle, being a civilian, usually leaves the dirty work to his detective friends. We are occasionally reminded that he is as good a shot as Beckett.
Momma's Boy: Raised solely by his single mother, and it shows in, among other things, his complete lack of knowledge about baseball.
Mystery Writer Detective: The premise of the series is Castle, a mystery writer, shadowing an NYPD homicide detective and eventually helping her solve mysteries. Later series have often had the detectives introduce Castle as a "consultant" suggesting he's been formally retained by the department.
Nice Guy: Once you scratch away the Playboy Man Child exterior, Castle is this through and through.
One of the Kids: He's quite childish for a grown man, but this has been heavily played down in later seasons to the point where he's shown more as simply retaining the adventuring nature of youth rather than acting childish.
Open-Minded Parent: Alexis is the one to set up and order her own punishment for jumping a turnstile, and demands he not let her go to the beach because she has to study. Sometimes, though, he can't quite be open-minded enough, like that one hippie boyfriend she had that drove him crazy.
Overprotective Dad: Alexis usually shuts him down before it goes too far, but she wasn't around to stop him from torturing a guy, because that guy was involved in her kidnapping.
Papa Wolf: Normally it's played for laughs, but in Season 5's Episode 15, "Target", when Alexis is involved in a kidnapping plot he gets scary. He tortured a man—an already injured man—for information.
Plucky Comic Relief: Despite being the star, he still fulfills this role for Beckett with his jokes and teasing and odd theories.
Suddenly Always Knew That: Castle is able to pull up incredibly esoteric knowledge to help nudge a case forward, which is justified in that he's a bestselling author of dozens of books that he researched, and researched well, before throwing that research out the window and writing whatever the hell he wanted. He is also often demonstrated to have a wide range of eclectic interests.
Supporting Protagonist: Despite being the focus of the show, Castle ticks about every other box on this list. He follows Beckett on her cases and provides help so she can solve them.
Uncle Pennybags: This has been downplayed but in earlier seasons he was happy to use his money and connections to help move a case along. It also appeared in other ways:
When he realized just how abysmal the precinct's coffee was, he gave them a new espresso maker as a gift.
When he realized what Beckett would do if she won a large lotto sum (help students who want to be lawyers if they follow her mother's path), he talks to a college about setting up the scholarship and donates personally to get it started.
All the times he's bribed Ryan and Esposito with court-side tickets to a basketball game.
He personally put up the fifty grand to pay for an assassination in the hopes of luring the murderer of Beckett's mother into the open.
Urban Legend Love Life: Despite the claims of his The Casanova charms, Castle has only been shown to be having sex with three women, two of whom were ex-wives (and one of those ex-wives is also his editor.) While Castle may enjoy the flirting and the dating, he doesn't seem too interested in the sexing part unless it's with someone he connects with.
UST: With Beckett. It became so obvious that Lanie joked that a corpse could see it.
Wacky Parent, Serious Child: The former with Alexis, the latter with Martha. Among the three of them, it's not clear at times who is raising who.
Wrong Genre Savvy: He's wrong far more often than he's right when it comes to his wild case theories, but he's also on the mark far more often than either he or Beckett would expect. Often it's clear that he's deliberately coming up with over-the-top or out there theories either because it would make a better story for him to write, because it would be really awesome if true, or because he wants to needle Beckett, rather than because he sincerely believes the theory is correct.
Brilliant police detective with an interest in and knack for solving left-field cases, and an overt exasperation for Castle's presence. She's a fan of Castle's novels and she does not want him to know this. He does.
The Ace: Youngest female detective in NYPD history, highest case closure rate, highly competent marksman and hand to hand combatant. Tremendously talented as an investigator and interrogator.
Broken Ace: She also has a wide variety of psychological issues, she's incredibly emotionally closed off, obsessed with her mothers murder and overall was portrayed as a miserable and lonely person before Castle showed up, and then she got shot and suffered from PTSD.
Action Girl: Police detective that specializes in tracking murderers.
Adorkable: Quirks like playing guitar and being a comic fan aside, the wink and raspberry she does at the end of "Swan Song" is truly something.
The Alleged Boss: Beckett is in charge of a team that loves her, but doesn't always obey her orders. Although she is the one giving the commands, the relationship between her, Ryan and Esposito is more that between friends and colleagues than that between a boss and her underlings. This is reinforced by the small age difference between them - while Becket is clearly more senior, she still looks and acts as a young woman. Summed up when Ryan and Esposito pull a risky and illegal stunt and are surprised when she chews them out.
Ascended Fangirl: Kate Beckett is secretly a big fan of Castle's, and Nikki Heat - the main character in his new books - is based on her. People mistook her for a character actress when she showed up for a book premiere.
Harming Castle. As she says to a mercenary threatening Castle during a hostage situation in a bank in "Cops and Robbers":
Beckett: "If you pull that trigger, I will walk through those doors and personally put a bullet through your skull."
Bringing up her mother's murder. It took nearly a year for her to be able to open up about it to Castle.
Brains and Bondage/Casual Kink: As with Castle, not shown on-screen but heavily implied. It is unclear, however, whether she just implies this to mess with Castle.
Brainy Brunette: Is portrayed a serious, professional and highly intellectual person who can trade literary references with Castle and legalese with lawyers, justified by her being in pre-law before her mothers murder.
She gets more blonde later on.
Broken Bird: Ever since her mother's death she's been a ball of pain and determination.
By-the-Book Cop: She is obsessed with professionalism and doing the right thing, all the times she breaks the rules are major character moments for her.
The Cast Showoff: Stana Katic got to take on a Russian accent in "Deep in Death"; Russian is one of several Slavic languages she speaks in real life.
Catch Phrase: "Castle!" with varying intonations, depending on the context.
Character Development: She's lightened up considerably over the years since Castle came along. However, thanks to her mother's case being reopened, she's also acquired new emotional (and physical) scars. One good example comes in "Swan Song". Season 1 Beckett would be closed off and focused on work, but in the denouement of the episode, she locks the camera crew in the closet with a wink and a raspberry.
Combat Stilettos: She has a propensity for four-inch heels. It is noted that she does this to give her a sense of dominance in interrogating people. A Running Gag is people asking her how she runs in them.
Control Freak: Beckett has been justifiably accused of being one of these. Lampshaded by Esposito in the pilot, referring to Castle following Beckett:
Esposito: A control freak like you, with something you can't control? No, no, that's gonna be more fun than Shark Week.
Cool Bike: As revealed in "Under the Gun", Beckett owns a Harley softtail.
Fair Cop: Lampshaded by Castle early on in the pilot, who comments that women with her looks and intelligence who are interested in fighting crime usually begin a career in law firms rather than pursuing police work. It becomes a meta-Shout-Out once we learn that Beckett's mother was a lawyer interested in helping those whom justice had overlooked and Beckett herself was planning be a lawyer until her mother's murder.
Fangirl: Of Castle. She would sooner die (or kill him) than have him find out. Too late. He already knows.
At work, Beckett presents a professional image and avoids dressing provocatively. Even in her private life she rarely shows any skin, and thus the rare instances where we get to see some, it's that much more memorable. Even then, it's usually Castle fantasizing, which usually ends with Beckett snarking mercilessly at him.
When she wants to fit in at an underground club frequented by Russian mobsters she ditches all her clothes but her red bra and panties, and then turns her jacket into an improvised dress that shows a lot of leg and Absolute Cleavage.
In one scene, Beckett is trying on clothes in front of Castle. We see her in her bra as she changes shirts because the first one looks too sexy for work. Castle then suggests that the second shirt isn't good either, but she realizes that he just wants to see her shirtless again.
The scenes where she shows the scar after being shot in the chest by a sniper, barely avoid being considered fanservice except maybe for the scene in "Always" where she finally admits her feelings for Castle and they kiss.
Green-Eyed Monster: For most of the series, it's subtle but clear that Beckett does not like it when other women grab Castle's attention.
Then along comes "Eye of the Beholder", when UST sparks between Castle and the insurance investigator attached to the case, which prompts Beckett to act like a seething fifteen-year-old girl. She does a poor job concealing her obvious jealousy for much of the episode.
She gets into a bit of a snit in "Pandora" when she meets Sophia Turner of the CIA, and discovers that she's not the first woman Castle has been inspired by as a 'muse'.
The Heroine: She's the main detective and, despite the name, she's the focus of the show.
Hidden Depths: There's lots of layers to the "Beckett onion", which Castle is frequently surprised to uncover, and which suggests a more fun-loving, lighthearted and even somewhat wilder personality than the seemingly no-nonsense workaholic cop front she projects; among other things, she owns a motorbike, alludes to interests in comic books, magic tricks and soap operas, among claims to have piercings and tattoos. However, several of these may may also be intended specifically to mess with Castle's head or to tease his obvious attraction to her. Several of them are also implied to have been part of her personality before her mother's murder caused her to become a lot harder and more serious, some of which have gradually revived the more time she's spent with Castle.
The Lad-ette: She's more intellectual and composed than the average Ladette, but she plays it up when messing with Castle, mentioning how she has a tattoo, did nude modeling, rides a motorcycle and frequently implies a rather kinky sex life.
Laser-Guided Amnesia: In "Rise", according to her, she remembers nothing following her shooting. Justified, because she was shot in the chest. Subverted because she's lying. She remembers everything, including Castle's anguished confession of love.
The Leader: She calls the shots in her little group.
Love at First Sight: Inverted. In "Still" she insists that she couldn't stand Castle when they first met.
Do you have any idea what you were like in the beginning?
Missing Mom: Johanna Beckett was murdered when Kate was studying pre-law at Stanford.
The Muse: To Castle; his latest protagonist, Nikki Heat, is based on her.
Old Shame: In Universe we have her brief career as a teenage model, revealed at the end of "Inventing the Girl". It was that or waitressing.
Only Sane Woman: Between Castle. Ryan and Esposito she's the most grounded.
Parental Substitute: Castle asks Beckett to be this to Alexis, if anything should ever happen to him (he got paranoid about being under a Mayan curse) in "Wrapped Up in Death".
Revenge: Beckett is clearly driven by this to some degree with regards to the murder of her mother; her every single major action or life decision since (joining the police department, becoming a homicide detective, etc) has been based on tracking down the person(s) responsible for the crime and bringing them down, to the point where any hint of her mother's case cropping up in an investigation is enough to drive her to the point of reckless single-minded obsession. The end of "Always", sees her have an epiphany which results in her decision to turn her back on this, instead realizing that all she really wants is a life with Castle.
Second Love: To Castle. His first real love was Kyra Blaine, introduced in "A Rose For Everafter."
Shell-Shocked Veteran: As of season four: per Word of God, she's suffering from PTSD. It's explored thoroughly during a case where the team try to find a killer who is a sniper.
Single-Issue Wonk: Most of the time, Beckett is a thoroughly rational, capable and methodical detective — unless the case relates to her mother's murder (or, as of Season 4, her own shooting), at which point she becomes increasingly irrational and out-of-control.
Statuesque Stunner: Not as obvious when she stands next to the 6'2" Nathan Fillion, but she's 5'9". She also likes to wear high heels which makes her look even taller.
Sugar and Ice Personality: Mostly with her interactions with Castle, where she vacillates between extreme cold professionalism and a laid-back wiseass attitude that loves volleying nerd jokes and lighthearted insults.
There Are No Therapists: Averted: it took a year of therapy for her in her backstory to stop obsessing over her mother's murder and to realize that she had a life of her own to live. As of season 4, she is also seeing a therapist on a regular basis to deal with the trauma of her near-fatal shooting. As well as to rant about her not-in-a-relationship problems with Castle.
The Tease: Beckett takes great amusement in leading on Castle's obvious attraction to her, only to bluntly shoot him down. Season 4 reveals that Beckett knew about his Anguished Declaration of Love in the Season 3 finale but lied about it, Castle convinces himself that she's just been leading him on for her own amusement and gives her the cold shoulder... which, as she's genuinely fallen in love with him by this point, makes things awkward.
In an early case, she discovers a father was very much likely the murderer of his son-in-law after the later killed the man's daughter but only she and Castle knew it. She could have let him go but choose to be lawful. She could also have allowed an assassin to kill her mother's murderer but being Lawful won again.
In "Need To Know" when she learned the CIA was going to force a distraught woman into being their mole within her mafia family or risk the CIA leaking that her boyfriend was a spy, Beckett chose Good by ignoring orders and leaking her name to the press as having ties to a crime family. This causes the family to cut ties with her out of self-preservation and ruins the woman as an asset. However, her bosses realize it was her and fire her for it.
Tsundere: Type A (tsun tsun); she's harsh and abrasive to many but has a softer side that only Castle can bring out. When he shows interest in other women she becomes intensely jealous yet refuses to admit it.
Undercover Model: In "Deep in Death", Beckett does this on the fly around Russian mobsters. More specifically: she ditches all clothing save for (bright red) lingerie and a jacket with Absolute Cleavage.
UST: With Castle. It became so obvious that Lanie joked that a corpse could see it.
Violently Protective Girlfriend: Once she and Castle become a couple, she becomes this. When 3xK tried to force their car into the river, she got out and returned fire with a Terminator-like expression.
When She Smiles: Beckett's face could light up the world when she smiles. The effect is particularly noticeable during the early seasons when Beckett is a mostly serious and grim character, making the moments when she expresses genuine joy all the more noticeable. Lampshaded by Castle during his Anguished Declaration of Love for her:
Castle: Every morning, I bring you a cup of coffee just so I can see a smile on your face.
Martha Rogers (Susan Sullivan)
Castle's flighty, emotional mother, who lives with him. A Broadway actress with a tendency towards the over-dramatic.
Fiery Redhead: Is a redhead and is able to keep Castle in line through force of personality.
Gold Digger: Martha is not one of these, despite her penchant for wealthy older men; she was, however, the victim of one, which forced her to move into her son and granddaughter's apartment. She's also not shy about charging vast amounts to her son's credit card, much to his exasperation, but it's made abundantly clear that she loves him for far more than his purchasing power. She also tries in "Lucky Stiff" to turn down the significant amount of money left to her by a deceased boyfriend.
Grandma, What Massive Hotness You Have: Despite her age, and thanks to her good looks and great personal charm, Martha is still attractive to much younger men. An example is Alexis' young, handsome violin teacher. Once he's met Martha, he is much more interested in teaching her than Alexis — to the disappointment of Alexis and the relief of Castle.
Mama Bear: In "Cops and Robbers" Martha has to be held back when it looks like one of the robbers is going to shoot Castle. In "Knockdown", she also gives Castle a smackdown when she thinks he's getting too cavalier about the danger he's in, telling him quite firmly that "...you can't charm your way out of a bullet!"
Pretty in Mink/It's Fake Fur, It's Fine: She has a penchant for animal-print and fur-lined clothing, and in "Lucky Stiff" goes on a shopping spree including buying a faux fur coat. She decides to return what she bought, except for the coat.
Castle: Is it safe to say there aren't any animal prints left in the city? Martha: Very funny. Also true.
Really Gets Around: In the pilot, she says that her son "never had a father figure" as an excuse for his misbehavior. His reply is that he had dozens of them.
Silver Fox: In all but the hair color, she's a well aged lady.
Wacky Parent, Serious Child: Castle is of course not a child anymore, and would normally be considered quite wacky himself, but compared to his mother's wackiness he is quite serious indeed.
White-Dwarf Starlet: Not completely burned out but she definitely doesn't get the roles she used to.
Alexis Castle (Molly C. Quinn)
Castle's teenage daughter who, against all odds, is grounded, sensible, mature, and very close to her father.
Damsel in Distress: In the fifth season episode "Target", Alexis is a victim in a kidnapping plot. She and her fellow victim actively try to manage the situation and escape.
Dating What Daddy Hates: Castle doesn't particularly appreciate Alexis dating anyone, but he's willing to play along so she can have those charming childhood moments. However, when she meets a hippy in Costa Rica and moves in with him in the sixth season... They eventually break up.
In an episode about teenage drug abuse, Castle confronts Alexis while she wakes up, telling her that she can tell him if she ever did anything wrong. She lies and tells him she's fine, but the next day she's so bothered by the lie that she wakes him up, crying... because once, she jumped the turnstile at the subway without paying. (Even then, she only did it because her card wasn't working and she wanted out of the rain.) She went the next day and paid twice without riding and still felt guilty about it.
She's gone back to a store and secretly paid after some of her friends used a Five-Finger Discount, despite the fact that she hadn't stolen anything herself.
The Intern: To Lanie, starting in the episode "Pandora".
Subverted in season 4, when she has applied for early admission to Stanford and everyone, including herself, thinks she will be accepted. She isn't. She takes this quite hard; her attitude basically is "Stanford or nothing" (justified because her boyfriend has been accepted).
She later gets acceptance letters from quite a few Ivy League universities, as well as Stanford. She chooses Columbia, and is studying there in season 5.
Like Father, Like Daughter: Not in personality, but in experience and skills. Though it's generally not shown off, Castle's many interests seem to have leaked into his daughter. She's shown interest and skill in everything from magic tricks to detective work and it's been established since the earlier episodes that most of Castle's epiphanies are discovered thanks to his habit of bouncing off his ideas on her. On the other hand, she's a lot more level headed and serious than her dad. She gets that from her grandfather.
Gets into Meta-levels in a Season 6 episode where Alexis and Castle work together to solve a case for her criminal justice class. The episode was literally titled "Like Father, like Daughter".
Morality Pet: For Castle, pre-Character Development at least; in the early seasons (and the first season especially), notice how Castle is a lot less of a douchey, arrogant playboy asshole whenever Alexis is around.
Missing Mom: Only in this case, as much as they care about her Castle and Alexis are kind of glad that Meredith isn't permanently around, as seen in "Always Buy Retail".
Raised by Dudes: Averted. There's little doubt that Alexis is a healthy, normal, intelligent, and well-rounded individual with much of the credit due to her father. Noted by Castle himself in "Lucky Stiff" that after getting over the 'just became extremely rich' phase of his career, he realized he only wanted two things in life: freedom to write and time to spend with Alexis.
She Is All Grown Up: Most evident/called out in "The Final Frontier" if only because she happens to appear in far more... revealing clothing than her usual modest fare.
Spoiled Sweet: Despite being raised in a very wealthy home, she is very down-to-Earth and kindhearted; a trait likely inherited from the happy-go-lucky Rick.
Suddenly Always Knew That: When she finds herself locked in a room, she takes advantage of the fact that her father taught her how to pick locks. Was he showing off research for a book?
Dashing Hispanic: Esposito is shown to be a little vain, and is sometimes shown to be asking women out, he has a decent on-screen record too.
Dead Partner: In "Den of Thieves," it is revealed that Esposito's partner before Ryan was Ike Thornton, killed by one of Victor Racine's men. It turns out that Ike was working undercover to take down Racine.
Friendly Sniper: During "Kill Shot". This and similar roles are justified through Espo's backstory of being in the Special Forces.
Friend to All Children: Especially troubled kids (as below). Esposito didn't have an easy childhood himself, and was a pretty serious juvenile delinquent. Naturally, he sees a lot of himself in some of the kids his work brings him into contact with, and usually takes an interest in them. He can also be terrifying when threatening the criminals that exploit those troubled kids.
Gentle Giant: Is a massive softie (especially for troubled kids) and all around Nice Guy, and yet is probably the most badass member of Beckett's investigative team.
Heterosexual Life-Partners: Ryan and him parallel the Beckett/Castle partnership in a lot of ways, and one of them is the jealousy they show when someone else is spending time with their partner.
Hidden Depths: At first he appears as another wisecracking cop but it is shown that he is very skilled and is very aware of Beckett and Castle's feelings for each other. He also appears to have numerous interests that conflict a bit with the stereotypically macho 'alpha-male' front he presents — he's surprisingly knowledgable on Top 40 bubblegum pop music, for example. He also seems to have a particular fondness for time-travel related science fiction; in "Time Will Tell", he namechecks Twelve Monkeys and The Terminator and — much to his embarrassment — accidentally reveals himself to be a fan of Doctor Who, and in "Punked" he is able to immediately identify a mock up of the flux capacitor inside a steampunk enthusiast's recreation of Doc Brown's Delorean.
Reformed Criminal: In "Under the Influence", Esposito reveals he committed a few crimes in his youth.
Sensitive Guy and Manly Man: Esposito is the first to make fun of Ryan for his Sickening Sweethearts interactions with Jenny, and is the simplest dressed of any of the main characters, usually wearing jeans and a long-sleeved shirt. Compared to Ryan, he's also more aggressive and stereotypically macho.
The Worf Effect: Most notably in "Knockout" and "Always", though justified. In "Always" Beckett does better than he does, because the special forces trained assassin targeted him first as he was the greater threat. In "Knockout" even Castle does better than him, though justified as he and Ryan were taken out with a flashbang.
Ryan: (while being tortured in ice water) "Listen, assclown. I went to Catholic school for twelve years. Hell, they use to do this to me for talking in class."
Berserk Button: As shown in "Kick the Ballistics", do not take Kevin Ryan's service weapon and especially do not use said weapon to kill an innocent young woman. He will find you and end you.
Beta Couple: With Jenny; their relationship is more stable than Beckett and Castle.
Butt Monkey: Mildly; Ryan seems to be the character who gets everything dumped on him. To his credit, he accepts it with good grace and a little snark.
S4 finale; after he told Gates about the case and how Beckett and Espo were investigating behind her back, leading them to disciplinary action, he's the only one left on the force, with Espo on administrative leave and Beckett resigned... and he and Esposito are no longer on speaking terms. Fortunately, this all ends up resolved after the first two episodes of S5.
Character Development: It's in the background but there — in the first season he was generally a bit scruffier and more slovenly in his personal dress and habits, but began dressing a lot smarter and getting more In Touch with His Feminine Side after getting seriously involved with Jenny. It probably helped that Castle entered the picture at the time and it's clear Castle is someone Ryan looks up to for direction (at least in this regard).
The Smart Guy: Calls himself the 'nerve center' of the group in "Swan Song".
Consider the extra characterization that you get in the Heat novels. His character Raley, is the "King of All Media" and is very good at finding/spotting/generally using surveillance as a tool. As Castle has noticed, he might not be as action-y as the rest of the cast, but he has his skills. He's usually the one to find the information that the entire team acts on, he just doesn't get as much credit.
Ryan has his shield and has demonstrated many times that he deserves it, but he's repeatedly the excuse for exposition, (again, "King of All Media") which, along with his Adorkable and Butt Monkey status implies that he's the least experienced member of the team. When his service weapon is stolen, and another cop calls him out on it, he later admits that the other cop did have a point, even telling a story of what he previously thought was his stupidest mistake as a cop.
Of the main four, he's tends to dress much more formally as expected of a detective, going with a jacket and type versus the more general professional look of Esposito and Beckett.
"The Wild Rover" establishes that he's been on homicide for 7 years (so 2 years at the start of the series) and an unspecified number working what amounts to major crimes. A lot of which is suggested to have been undercover and in tandem with the local FBI's organized crime unit. So while he may or may not be the least experienced, it's certainly relative.
Happily Married: To Jenny, whom he began dating in season 1. They get married in the season 4 episode "Till Death Do Us Part" and now have a kid.
The Heart: He seems to be the most outwardly sensitive of the detectives.
Hero-Worshipper: He's the most likely of the detectives to consider Ricks wild musings.
Ryan feels no shame in making comments like "[Castle] really is ruggedly handsome..." when he sees a cardboard cutout of the former and notes that he "feels like he's being cheated on" while Castle works with Slaughter.
Buxom Is Better: Yes and no. She mentioned in one episode how she always wanted to be a ballerina as a child, but had to stop when she turned 13 and "the girls" made her too top-heavy to continue. Since then, she decided they have their perks.
The captain of the unit. Made of complete and utter awesome.
The Atoner: As revealed in the season 3 finale "Knockout", he was a part of the conspiracy that led to Beckett's mother's death. He met her in the records room going over the case's evidence, and took her under his wing, guiding her to be the head detective of the 12th Precinct. He finally earns his redemption by taking out Hal Lockwood, the man who works for the man who ordered Beckett's mother's death, at the cost of his own life.
A Father to His Men: Treats his detectives, and especially Beckett as kids. He always tries to protect them from political fallout.
Benevolent Boss: While he does have his scary moments, overall Roy is pretty laid back about rules and deeply cares for the personal well-being of his cops. He also has no problem responding to the goofy questions Castle asks. This is especially noticeable when you compare him to his replacement, Captain Gates.
Big Good: He's Beckett's boss which makes him the head good guy.
Last Stand: Died kicking ass despite being outnumbered.
Reasonable Authority Figure: Heck, even in "Knockdown" after (rightfully) pulling Beckett off the case, he's willing to listen to her and is more concerned with her safety when it's apparent she's in danger and not going to quit.
Retirony: Played with - spectacularly - in a throwaway gag. The Captain promises his wife that he'll retire the following year. When Castle shows concern, Beckett explains that Montgomery retires "all the time. Just give it a week or two. He's like the Brett Favre of the NYPD. Trust me - he's not going anywhere." He dies the next week.
Walking Spoiler: Look at all those blanked out lines. Don't uncover them if you haven't watched through the third season.
Victoria "Iron" Gates
Victoria "Iron" Gates (Penny Johnson Jerald)
The precinct's new captain after the death of Captain Montgomery. Unlike her predecessor, she is not happy about having Castle tag along with her cops but puts up with him due to his connections with the mayor. Also contrasting to Roy is their attitude towards their superiors. Roy kept Castle around primarily because he thought Castle was a good man and protected his people regardless of political fallout. Gates, on the other hand, tends to want to cultivate a positive image of her people and seems partially motivated by the fact that she's a woman in a traditionally man's profession and rank. Because of this she often tends to be pressured by the department's PR or her superiors - which also means that of her initial tolerance of Castle was only because she was told she had to. This also means that she often bounces between like and dislike of Castle depending on whether or not he's goofing around.
Bait-and-Switch Tyrant: Played with; she's no Roy Montgomery, but she seems to be softening towards Castle somewhat and is a dedicated officer who is very good at her job.
Defrosting Ice Queen: Towards Castle. At first, she pretty much thought him a liability, but by mid-season 4 she already regarded him as one of her people. She even aids him in "Probable Cause", as even she doesn't believe all the evidence pointing to him. Castle seems to be returning the favor, for his part, addressing her as Sir, as per her request. There are moments where she starts genuinely liking Castle, but he always manages to screw those up and leave her hating him even more than before.
Foil: To Montgomery: He kept Castle around because he wanted to and thought he would be a good influence on Beckett. Gates is forced to keep Castle around by the mayor, and thinks (at first anyway) that Kate would be a better dectective without him.
Genre Savvy: Gates is no fool. As an IA detective, it was her job to hunt out internal wrongs and notice corruption.
She was able to put together Kate and Rick dating.
She realized the only reason three morally strong detectives lied about the investigation into Kate's shooting is it had to deal with Capt. Montgomery's death and likely bad things he had done, and they wanted to protect his name.
Hidden Depths: Season 5's Secret's Safe With Me has the big reveal. She collects dolls! She also has a weakness for reality shows.
Mama Bear: She is extremely protective of her detectives, and though she doesn't hesitate to make them play politics, she will also step in and take the heat for them without hesitation if she believes that they are right and the higher-ups are wrong.
She, much like the rest of the main cast, could see Castle and Beckett were in an intimate relationship for much of Season 5. She just never chose to address it as she had this and she still will as long as the two don't act out at or slack off at work.
She stops Beckett short of saying that Montgomery had done some wrong things in the past.
Pointy-Haired Boss: Her new subordinates view her like this initially. While she's not incompetent, it is justified in some respects — in particular, she tends to let her dislike of Castle distort her perception of how useful he is, and tends to dismiss his contributions simply because he's the one giving them. But over time she becomes a...
Reasonable Authority Figure: She is stern, cold, and strongly dislikes Castle but an all-out competent boss who is willing to listen to Castle when his plans make sense and is someone who will not let her people slack off in the moral sector. She also tries to keep Beckett from being too reliant on Castle. She also takes umbrage with the Feds taking over her place using only the phrase "Need to know" as explaining why, knowing that they care about the big picture and not the little guy.
Shipping Torpedo: Subverted. when everyone assumes Gates would flip out if she discovered Rick and Kate's relationship it turns out she's as big a Shipper on Deck as the rest of the cast, as long as they maintain professional behavior on the job.
Tyrant Takes the Helm: In comparison to Montgomery, Victoria "Iron" Gates fits this trope to a T. As an example of her more straight laced approach, where as Montgomery made no fuss about people calling him Roy in the workplace, Gates asks that everyone - even Castle who technically doesn't have to - call her Sir.
In the first few episodes of Season 4, she questioned the compentecy of her detectives when investigations didn't quite go as she was hoping, like the viligante super-hero episode.
Vitriolic Best Buds: "Best buds" may be stretching it (a lot), but as season 5—particularly the finale—shows, Gates has come to at least tolerate if not like Castle. She still won't pass up a chance to troll him or chew him out whenever she can—and that's probably more due to his goofiness compared to her more straitlaced personality than actual dislike.
M.E. Sidney Perlmutter
M.E. Sidney Perlmutter (Arye Gross)
A second coroner introduced in season two. Grumpy and snarky. Where as Lanie will snark at Castle in a friendly manner, Perlmutter will simply snark at Castle in general.
Meredith (Darby Stanchfield): Castle's first ex-wife and Alexis' mother, introduced in "Always Buy Retail". Though they love her, they find her impulsive, irresponsible personality annoying. She works as an actress and while she shares many similarities with Martha, the two of them dislike each other. Castle describes her as a "deep-fried Twinkie" because though he knows she is bad for him, he still wants to enjoy one occasionally.
Gina Cowell (Monet Mazur): Castle's second ex-wife and publisher. They are generally on good terms so long as they don't try to restart their relationship.
Kyra Blaine (Alyssa Milano): One of Castle's ex-girlfriends (and his first love, to boot) who is reunited with him when a guest at her wedding is murdered in "A Rose For Everafter". Believes, like so many others, that Beckett and Castle are a couple, is a total Shipper on Deck and out-loud gives Beckett her blessing.
Johanna Beckett: Kate Beckett's mother and a case of Death by Origin Story. She was murdered and her killer was not found due to incompetent police work (which turned out to be not so much incompetent as downright dirty). Eventually Beckett finds him and shoots him, but the wider conspiracy involving Senator Bracken while solved is unprovable, leaving Kate in a Mexican Standoff with the senator. Kate wears her mother's wedding ring on a necklace.
Jim Beckett (Scott Paulin): Kate Beckett's father. Fell into alcoholism after the death of his wife but recovered thanks to the help of his daughter. Kate now wears his watch to remember the life she saved. The watch is damaged when Beckett's apartment is blown up in "Boom!" but Castle has it repaired for her as a surprise. When Beckett begins investigating her mother's death again, in "Knockout", he tries to get her to stop and enlists Castle's help.
Josh Davidson (Victor Webster): Beckett's boyfriend, first introduced at the end of "Punked." They break up after Beckett is shot.
Jenny O'Malley (Juliana Dever): Detective Ryan's Love Interest. Frequently mentioned but appears onscreen only rarely, played by Seamus Dever's real-life wife Juliana. Becomes his fiancee in the episode "Nikki Heat" and marries him immediately after "Till Death Do Us Part."
Madison Queller (Julia Gonzalo): An old friend of Beckett who briefly dates Castle, stirring some jealousy in Beckett. She is the owner of a restaurant where a chef is murdered, as shown in "Food to Die For".
Natalie Rhodes (Laura Prepon): The actress who is cast to play Nikki Heat in the upcoming Heat Wave movie, as seen in the episode "Nikki Heat". Castle initially has doubts about her due to her resume of low-brow action and horror movies but soon warms up to her. A method actress, she shadows Beckett and imitates her mannerisms and appearance, which she is unnervingly good at. As character research, she insists on doing everything that Nikki does. Everything.
Damian Westlake (Jason Wiles): An old friend of Castle's from boarding school, shown in "The Final Nail". He was the editor of the literary magazine, where he became Castle's first mentor. He was accused of his father's murder but the evidence was inconclusive and he inherited his father's wealth. Damian's father was murdered by one of Damian's classmates but, in a last-minute twist, it's revealed that Damian hired the classmate to kill him.
Alex Conrad (Brendan Hines): A young mystery writer who just published his first novel with help from Castle, seen in "The Dead Pool". He begins shadowing Beckett, which makes Castle jealous. Castle invites Conrad to one of his poker games, where his fellow writers engage in some "friendly hazing." Afterwards, Conrad begins hanging out with Ryan and Esposito. It is implied they become his new "muses."
Evelyn Montgomery (Judith Scott): Roy Montgomery's wife, introduced in "Pretty Dead". They have two daughters named Mary (LaNiesha Irvin) and Rebecca (Daniele Watts).
Ashley (Ken Baumann): Alexis' boyfriend, first introduced in "Punked". She makes use of his Gender-Blender Name to trick her father into letting him come over, which led to a rather...awkward first meeting between father and suitor. After he graduates at the end of Season 3, he goes to Stanford, and while Alexis wants to maintain a long-distance relationship until she can graduate early and move out with him, there seem to be a few... snags... in Season 4, until ultimately the stresses of a long-distance relationship proved too much and Alexis broke up with him.
Robert Weldon (Joseph C. Phillips): The mayor of New York City and a close friend of Richard Castle. He is the one who initially gets Castle the chance to shadow Beckett. He is investigated for embezzling money from a charity. While blame ultimately falls to one of his staffers, the scandal keeps him from running for governor as he originally planned.
"Jackson Hunt" (James Brolin): The alias of Castle's father, a spy. He'd planned to settle down with Martha but circumstances forced him to leave the country and cut off all contact with her. Aids Castle in rescuing his daughter (and Hunt's granddaughter) from the clutches of Hunt's archenemy, Gregor Volkov.
Papa Wolf: Exploiting this trait toward his son Richard and granddaughter Alexis is how his archenemy Gregor Volkov got him to come out of hiding.
Pi (Myko Olivier): Another of Alexis' boyfriends, who she meets in college. Castle disapproves of his New-Age Retro Hippie tendencies as when the guy was staying at his home, rarely cleaned up after himself, would walk in on Castle and Beckett without warning, and they leave him with few job prospects.
Det. Tom Demming (Michael Trucco): An NYPD robbery detective, first seen in "Den of Thieves", who becomes Beckett's boyfriend until their breakup at the end of the second season.
Dr. Carter Burke (Michael Dorn): Beckett's psychiatrist in Season 5.
Agent Will Sorenson (Bailey Chase): An FBI agent specializing in missing children and Beckett's ex-boyfriend, as revealed in "Little Girl Lost".
Agent Jordan Shaw (Dana Delaney): An FBI profiler who helps capture a crazed killer targeting "Nikki Heat" in the season 2 two-parter "Tick, Tick, Tick..."/"Boom!" Her skills and fancy equipment impress Castle, stirring feelings of jealousy in Beckett. However, she too comes to respect Agent Shaw.
Ike Thornton (Aaron Spears): Esposito's old partner, as seen in "Den of Thieves", who turned to a life of crime and left his family behind. It turns out that Thornton was infiltrating a crime ring and helping bring down a powerful mob boss.
Clark Murray (Robert Picardo): Known to Castle as "Dr. Death", he aids Castle by showing him the physics of dead bodies so he can get them right in his novels. Castle asks him to look into Beckett's mother's murder behind her back; later, he ties the victim of the week in "Sucker Punch" to her mother's killer, Dick Coonan.
Mike Royce (Jason Beghe): A cop turned bounty hunter and Beckett's old mentor, introduced in "Under the Gun". He then betrays his old ideals and tries to steal a hidden hoard of jewels, resulting in a Broken Pedestal. He comes Back for the Dead in "To Love and Die in L.A.", prompting Beckett to chase his killer to Hollywood.
Mark Fallon (Adrian Pasdar): Seen in the season 3 two-parter "Setup"/"Countdown", a Homeland Security agent who has been compared to Jack Bauer. His harsh interrogation methods and use of racial profiling put him at odds with the main characters. He's a bit of a Jerkass Woobie when you find out his backstory. His wife's death on 9/11 is the primary reason for his cold tactics and harsh attitude in the interrogation room. To make it worse, he and his wife were on the phone when the towers collapsed.
Sophia Turner (Jennifer Beals): A CIA agent who appears in "Pandora"/"Linchpin". She inspired the character of 'Clara Strike' in Richard Castle's 'Derrick Storm' series of novels, and they also shared more than a bit of romantic chemistry, much to Beckett's annoyance. She turns out to be The Mole. She also hints that she knew Castle's father but she is killed before she can say more.
Detective Slaughter (Adam Baldwin): Featured in "Headhunters", a Cowboy Cop that Castle works with when he is falling out of touch with Beckett. Surprisingly they avoid any Firefly references (well, maybe the brown jacket).
Ann Hastings (Valerie Azlynn): NYPD officer in the 12th Precinct. Revealed to be the vigilante "superhero" Lone Justice. Fangirl of Beckett, and allows Beckett access to Senator Bracken while working his security detail.
LT: An otherwise unnamed officer who rarely speaks in the series. But he's shown up multiple times helping on the case of the week or guarding crime scenes. Castle interacts with him the most. He finally speaks in Veritas, and he trusts the group consisting of Castle, Beckett, Esposito, and Ryan enough to cover them in a tight spot.
Agent Harris (Dylan Walsh): FBI agent brought in to oversee the safe return of Sara El-Masri to her parents (and, eventually, Alexis to Castle.)
Rachel Mc Cord (Lisa Edelstein): Beckett's partner in DC. She is much more cynical and accepting of the moral compromises that being a special agent for the DOJ requires than Beckett is.
Tory Ellis (Maya Stojan): NYPD computer tech whose appearances are limited to explaining whatever tech is involved in the case of the week.
Dick Coonan (Jay Ferguson): A corrupt, drug-trafficking philanthropist who appears in "Sucker Punch." He is the man who was hired to kill Beckett's mother. He is killed by Beckett while taking Castle hostage, the identity of his employer apparently dying with him.
Scott Dunn (Dameon Clarke): A serial killer who appears in "Tick Tick Tick..." and "Boom!" He commits several murders in New York "dedicated" to Kate Beckett, who he thinks of as Nikki Heat, and tries to frame a man named Ben Conrad. He commemorates his murders by writing novels based on them, including his Nikki Heat fanfic "Dead Heat." It is implied he committed another series of murders against prostitutes in Seattle.
Triple Killer/3XK/Jerry Tyson (Michael Moseley): A serial killer who targets women, killing three within the same week. The name has a double meaning as there are, in fact, three people involved in the killings. The original killer, Jerry Tyson, pays two friends to help him. Because one of the two friends needs an operation, they agree. The only killer in the series so far who escapes. Revisited in the season 4 episode "Kick the Ballistics", where the gun he stole from Ryan reappears in a recent murder case, which turns out to have been committed by a man Jerry Tyson befriended during his incarceration. Reappears in season 5 episode "Probable Cause", where he frames Castle for a murder that has been built to look like one in a first draft for one of his novels, and it turns out he has been stalking Castle for quite some time. Apparently dies after being shot and falling down the bridge where he has confronted Castle and Beckett, but it is implied he may have done this to disappear again and keep killing.
The Chessmaster: In "Probable Cause" is one big Xanatos Gambit. And he pulls it off, again.
Genre Savvy: In "3XK", Beckett called Castle about the case in the hopes it would lead to a Eureka Moment because she was stumped and three-quarters into the episode. Castle in the same scene because he calls her out on it. Borders on Lampshade Hanging.
Impersonating an Officer: In "Probable Cause" Serial Killer 3XK, dressed as a cop, visits Castle in the precinct's holding area to reveal that he's the one who framed the writer for the Murder of the Week.
It's Personal: In "Kick The Ballistics", it's discovered that the gun used in the murder of a college student was the same gun that the 3XK Killer stole from Ryan in the previous season. When he learns this, Ryan takes it hard.
Myth Arc: The center of one of these in the show, the other being the murder of Beckett's mother.
Never Found the Body: In "Probable Cause" so Castle is immediately convinced that the death was faked, but Beckett is less convinced. The trope is finally confirmed (i.e. 3XK is still alive) as of "Disciple".
Hal Lockwood (Max Martini): The alias used by the hit man, introduced in "Knockdown", who shot Detective Raglan, who led the investigation of Johanna Beckett's murder. He was imprisoned but refused to give any information on his employer. Beckett vowed to visit him every week until he decided to tell her what she wants to know. Used dirty cop connections to get himself into General Population, where he killed the other detective who was on that case, leading to an arraignment where his associates busted him free. This led to a series of events that ended with him being killed by Captain Montgomery in his final act.
Affably Evil: Quite polite and professional in the execution of his work, even assassinations and torture.
Badass: Has one of the highest kill counts in the series, and unlike the other mass-murderer, the 3XK Killer, Hal kills combat trained police. He does a helicopter escape from a courthouse and other acts more suited to a Bourne movie than an otherwise fairly reasonable show.
Knight of Cerebus: While the Beckett's Mom Arc has always been a darker one, Hal's appearances cause massive emotional trauma to the main characters. He's also the first sign that there is a massive conspiracy and cover-up around Beckett's Mom, and he kills Captain Montgomery.
Man In Black: An unconventional one, but he doesn't express emotion, is an agent of a mysterious conspiracy involving the government, is dedicated to covering up any evidence of that conspiracy and silencing any that are seeking its secrets and wears clothing from a dark color palette.
Perma Stubble: Lockwood has a few's days growth on his face in all of his appearances, in fact it appears to be at exactly the same length, which considering that one of those appearances was in prison is a pretty impressive feat. Also, considering that Lockwood's scruff makes him more distinctive, a handicap for a pro killer, the only reason for the beard seems to be adding to his Obviously Evil nature.
Professional Killer: While it's never explicitly stated, he doesn't appear to have any hatred towards his victims, nor does he express a loyalty to a cause greater than himself, so by process of elimination he's one of these.
The Stoic: Doesn't raise his voice above a medium rasp.
Cole Maddox (Tahmoh Penikett): Assumed name of a killer who, in "Always", is later revealed to be the sniper who shot Beckett. When Esposito and Beckett catch up to him, he proceeds to take the both of them down with ease, knocking Esposito out cold and leaving Beckett hanging off the edge of a building. His mission is to find Montgomery's contact who received information damaging to his employer. At the end of season 4, he has found Montgomery's contact (Mr. Smith), and tells him that he still has every intention of killing Beckett. After destroying the original file, he tails Castle and Beckett to the location of a hidden backup file, then cuffs them both (not to each other this time) to kill later...after he got the backup. He did. Unfortunately for him, the file was booby-trapped. Kablooey.
Senator William Bracken (Jack Coleman): Protected New York Senator with aspirations of much higher office. He was NYC's district attorney in the late 80s, which put him directly in the front row of the mob ransom scenario. He couldn't get the mob guys to testify against the cops or stop the mob because the system was so corrupt, so he decided he'd get a piece of the pie and blackmailed the corrupt police officers into giving him their ill-gotten gains. He uses this to fund his first campaign, but needs to cover up where the funds came from as he's looking to go much much farther in politics. When Johanna Beckett starts investigating the original crime that stopped the corrupt cops, she pulls on a very long and tenuous thread and thus, he is the man that is most responsible for Beckett's mother's murder, despite not pulling the trigger. And all the ensuing cover up murders that followed. Also, he could have had more help, as it is mentioned, quite frequently that he is untouchable. Beckett finally manages to arrest him in "Veritas", when she finds proof where the man admits to several crimes.
Affably Evil: Has polite small talk with Beckett and the crew, even though they've both planned on killing each other.
Big Bad: Serves as this for the entire 'Beckett's mother arc', having been the politician who ordered her murder.
Gregor Volkov (Nestor Serrano): Internationally-known crime boss, known enough for Richard Castle to fictionalize him in his Derrick Storm series of novels. Archenemy of Jackson Hunt, Castle's father. Kidnaps Alexis after he learns she's his granddaughter, and kidnaps her friend, an Egyptian heiress, along with her to get his attention. He's killed by Hunt through Castle's stolen radio-bomb in their rescue attempt and assault on his compound.
Vulcan Simmons (Jonathan Adams): A dealer that lived in the area that Beckett's mother had been visiting and reaching out to. Though not actually involved in the murder itself, he thought that it was deserved, prompting Beckett to try to put him through the observation glass at the precinct. While not a initially a direct threat, he graduates into full villain territory in Season 6, when he is revealed to be working to build the financial aspects of Senator Bracken's operations before he runs for president, and nearly kills Beckett during an interrogation as payback for their initial encounter. He's killed in "Veritas" by someone who plants the killing on Beckett.
Dr. Kelly Nieman (Annie Wersching): Plastic surgeon and former prison doctor who fell in love with Jerry Tyson while she was in prison. She performed Magic Plastic Surgery on two people to look like Lainie and Esposito so she could access the NYPD archives and medical records to remove all traces of 3XK from the system for an as-yet unknown purpose.
Real Life Mystery Novelists at Castle's Poker Games
Michael Connelly: Most famous for the Harry Bosch series, which is mentioned in the show.
Dennis Lehane: Wrote the Kenzie-Gennaro series.
Other Famous Cameos
Joe Torre in "Suicide Squeeze"
Rocco Dispirito in "Food to Die For"
Gene Simmons in "To Love and Die in L.A."
Wes Craven in "Scared to Death"
Michael Smith (Geoff Pierson): The recipient of Montgomery's letter designed to stop the attacks on Beckett. In "Rise", he contacts Castle and advises him to get Beckett to back off her investigation into her mother's murder, as they will try to kill her again if she presses. In "Dial M for Mayor", he is assumed to have resolved the case in such a way that the Mayor's involvement is kept secret, because they need the Mayor in place to keep Gates from removing Castle from the 12th precinct... a move that would have disastrous consequences for Beckett. In "Always", he is discovered by Cole Maddox, a killer hired by the person behind Beckett's mother's murder. He's tortured (but not killed) by Maddox in an attempt to destroy the evidence against his employer. He gives Castle and Beckett clues to the location of a secret backup of the file, allowing Maddox to get his hands on it (and get himself blown to smithereens by the trap set by Smith). Smith decides to quit himself of Beckett and "disappear" after this, but he's killed by (presumably) an associate of the Big Bad. However, late in season 6 he turns out, with the help of his doctor, to have been Faking the Dead and informs Beckett of a crucial piece of evidence that can bring down Bracken before going back into hiding.