Beckett’s initial relationship with Castle was strained because she viewed him as immature and reckless. She suspected (correctly) that Castle was using his “research” as an excuse to pursue her, and only tolerated his presence at the request of Captain Montgomery. While she came to respect the value of his unique perspective on crime, she remained frustrated with him on a personal level. This frustration reached a boiling point when Castle, against Beckett’s wishes, re-opened her mother’s murder case. She was angry and hurt, and requested that he end their relationship. Only when he apologized and agreed to end the partnership did she relent and offer to let him stay.
Castle continues to pursue Beckett, while she still fends him off. However, the flirtation is now two-way, and there are signs that she sees Castle as more than just a friend. While hotly denying it, she is clearly grief-stricken in “When the Bough Breaks” when it seems that Castle will be ending their partnership now that he’s finished writing Heat Wave.
In A Rose for Everafter“, Beckett struggles with her burgeoning feelings for Castle when he reunites with an old flame. Kyra Blaine is not what Beckett would have expected: as well as her looks, she’s real and, as Beckett observes to Castle, “I didn’t think you went for real.” She is also clearly jealous of Castle’s feelings for her. We also see Castle from a new perspective. He was heartbroken when Kyra never spoke to him again after a “break” in their relationship, and this puts the more shallow women he’s been with in a new light. Like Beckett, emotionally isolated after the loss of her mother, Castle avoids taking emotional risks by staying in superficial relationships.
The first major turning point in their relationship comes in Sucker Punch, when Castle spent $100,000 of his own money to catch the hit man who killed her mother. Castle was taken hostage when their informant turned out to be the hit man himself, and Beckett was forced to kill him to save Castle. Dick Coonan died before revealing who hired him, and Castle, blaming himself, offered to end their partnership. Beckett asks him to stay. She explains that she’s “gotten used to you pulling my pigtails” and that he’s made her job more fun. From then on, the banter between them becomes more affectionate and playful, and a genuine friendship develops.
In Tick, Tick, Tick…, Beckett’s jealousy is professional, not romantic, as Castle’s attention focuses on Agent Shaw and her high-tech equipment. Agent Shaw immediately sees the chemistry between the two and assumes that they’re already sleeping together. In the same episode, he is seen to be worried about Beckett, even staying in her apartment to protect her from a murderer. He made her pancakes in the morning, which she was teased about by Ryan and Esposito. During the aftermath of her apartment being blown up, Beckett stays at Castle’s apartment, where she makes him breakfast.
At the end of season 2, Beckett develops a romance with detective Tom Demming. This relationship becomes increasingly serious, and Castle’s jealousy makes him increasingly hostile and competitive with Demming. In Food to Die For, Castle and Beckett discuss their victim, killer and the girl caught in between. Castle says that the girl should have gone with the one she obviously truly loved, and Beckett argues that she saw the logic in staying with the one that seemed responsible and more dependable at that time, as opposed to the one with a history as a playboy that makes you feel good for a while, but has a risk of just letting you down eventually. Castle replies that she should have just followed what the heart wanted since things would have turned out well for them as the former playboy turned out to have been planning a proposal before he died. The conversation is their indirect reference to the current situation between themselves and Demming.
In A Deadly Game, Castle invites Beckett along on his annual Memorial Day trip to the Hamptons. Beckett refuses, claiming she is busy, but it quickly becomes clear that she has plans with Demmming. Realizing that he’s lost, Castle ends the partnership on the pretext that he needs to finish his book. While he claims he will be gone for the summer, Beckett realizes that this might be a permanent parting. Unknown to Castle, Beckett has re-examined her feelings for him. Desperate not to lose him, she breaks up with Demming and approaches Castle to declare her own feelings for him. But it’s too late; Beckett was on the brink of telling Castle about her feelings when his ex-wife Gina showed up to go to the Hamptons with him. Castle explains this, believing that he’s gracefully stepping aside, but this crushes Kate, who waited too long and took an emotional risk for nothing. They do not share any communication whatsoever during the summer.
Castle returns after the summer, but does not call Beckett or the precinct. The detectives, especially Beckett, feel snubbed, and she assumes that he didn’t call because he doesn’t really care about her. Castle, for his part, assumes that she’s still with Demming and has procrastinated about contacting Beckett again because of his unrequited feelings and a sense that things might not be the same. They are forced together when the detectives arrive at a murder scene to find Castle standing over a dead body holding a gun. Though he explains the victim was an acquaintance that called him for help, he is arrested for murder and interrogated by Beckett at the precinct. Stung by the snub, Beckett doesn’t want Castle back, but he eventually regains her trust. Things proceed much as in Season Two: Castle and Beckett are friends, and while each wants something more, outside events and their own emotional issues keep them apart. The two enjoy witty banter and unresolved sexual tension, but without the fulfillment of actually engaging in a relationship. Eventually, she comes to refer to him as her partner, tacitly acknowledging that the two are more than just friends.
Nevertheless, the two continue to pursue relationships with other people. Castle remains with his ex-wife Gina, though the relationship remains stormy and the two break up. However, by this time, Beckett is now in a relationship with Josh, a philanthropic doctor, whom she remains with through the rest of the season.
In 3XK“, Castle is left in a hotel room tied to a chair by serial killer Jerry Tyson. The closing scenes of this episode clearly show their emotional connection: Beckett’s terror that Castle might be dead, and then her compassion when she sees how depressed he is over his failure. The two hold hands for the first time.
In Knockdown“, Castle and Beckett finally kiss for the first time. Although this is a ploy to distract a guard and save Ryan and Esposito, it’s clearly seen by both their faces that they felt something more. In Setup, Beckett acknowledges that she and Josh are having problems, stemming from a sense that he’s not really there for her. Later, in the same episode, Castle and Beckett are trapped in a freezer. Huddled together and slowly dying of hypothermia, the two express the depth of their commitment to one another, and Beckett very nearly tells Castle that she loves him, but passes out just before she can finish her sentence. Castle wakes to find Beckett being treated by Josh, who she says she thinks she has a chance with. Later, Castle is about to admit his feelings for her, but Josh walks in, and he leaves instead.
In To Love and Die in L.A., the two are staying together in a hotel room, where Castle tells Beckett how much he admires and adores her. She says he’s not so bad himself. They look at each other for several seconds, but as the tension builds, she backs down and walks out, terrified. In her room, she collects herself and goes back out to talk to him, but he’s already gone to bed. In Knockout, Castle must choose between his relationship with Beckett and saving her life. Montgomery and Beckett’s own father urge him to talk to her, to tell her to stop pursuing her mother’s case. Castle confronts her, and the heated conversation segues to the question of exactly what their relationship really is. He also accuses her of “hiding” in relationships with men she doesn’t love. The argument escalates, and she throws him out, ending the partnership. While delivering a eulogy at Montgomery’s funeral, Castle sees the sun reflecting off what he realizes to be a sniper rifle that is aimed at Beckett. Before he can tackle her out of the way, Beckett is shot in the chest. As she’s laying on the ground, Castle begs her to stay with him and confesses his love for her, telling her “I love you, Kate” before she passes out with tears in her eyes.
Beckett survives her injury after undergoing extensive surgery and even flatlining. When Castle visits her, she claims to have lost all memory of what happened and asks him to give her some time to process everything. However, a “couple of days” turns into three months and she still doesn’t call Castle during that time. The new captain, Victoria Gates, is hostile to Castle and throws him out of the precinct. When Beckett finally contacts Castle at a book signing, clearly surprising him, he is at first bitter toward her for not contacting him and is reluctant to return. He only finally makes peace with her when she mentions that she and Josh broke up. She tells Castle that she feels she can’t have an actual relationship while the people that had her mother killed are still free.
In Eye of the Beholder, Beckett becomes jealous when Castle becomes interested in another woman, but refuses to acknowledge her feelings towards Castle, leading her psychologist to question whether she’s worried that he won’t wait for her… or that he will.
Cops & Robbers features some very meaningful looks when Beckett rescues Castle from a hostage situation at a bank, but again they do not explicitly acknowledge their feelings although they are more than clear. In Cuffed, the two spend the entire episode handcuffed to one another and the episode ends with Castle saying that there was no one he’d rather be “hitched” to. Beckett agrees, but suggests that they “leave out the tiger next time”.
As Beckett begins to entertain the possibility of a romance with her partner, Castle’s own fears begin to take center stage. In 47 Seconds, he inadvertently discovers definitive proof that Beckett did in fact hear and remember his confession of love and has been lying for seven months about it. Castle is stunned and obviously hurt that Beckett has been lying to him the entire time about not remembering anything. Castle takes this as meaning that she doesn’t have feelings for him and was letting him down easy by pretending she hadn’t heard. Castle resolves to make their relationship purely professional. He begins a string of casual affairs, returning to his playboy past. When British detective Colin Hunt teams up with Castle and Beckett, Castle ignores the rival and encourages the two to work together. Ironically, this comes right when Beckett finally confesses her feelings for Castle to Lanie. Lanie had warned that Castle wouldn’t wait forever for Beckett, and now it seems she has finally chosen to act too late.
While Beckett remains surprised and hurt that Castle has suddenly begun to distance himself from her, she still has not figured out what’s triggered this abrupt change. In Undead Again, the case Beckett and Castle are working on partly mirrors their own problems such as one witness claiming not to remember anything of a murder. In the end, Beckett tells Castle that she is seeing a therapist and is working on getting herself to a place where she can handle everything that happened the day she was shot. She admits the wall inside her is slowly coming down. Castle says “I’d like to be there when it does” to which Beckett replies “Yeah, I’d like you to be there, too”.
Always is in many ways a reprise of Knockdown. A lead in the killing of Beckett’s mom comes when the mysterious killer acts to cover up his crime. As the body count rises and the size of the conspiracy becomes evident, Castle confronts Beckett and insists that she abandons the case to save her own life. This time, he finally reveals his own dirty secret; he has participated in the cover-up to protect her. When Beckett angrily demands to know why he would do that, he finally confesses his love openly to her and asks her to choose between him and the case which has defined her, but Beckett refuses to put the case behind her and says it is her life to live. Castle allows her to make her choice, but says that he refuses to watch her throw her life away and ends their partnership before leaving. Beckett fails to catch the killer and is left literally hanging from the edge of a building. Beckett, facing suspension for misconduct in her handling of the case, chooses to resign instead. That night during a heavy rainstorm, she returns to an initially resistant Castle desperate to say how sorry she is. Kate explains how Maddox got away, and that she nearly died but all she could think about was him and says to Castle, “I just want you”. The two begin to kiss passionately before they go to Castle’s bedroom together and make love for the first time.
After the Storm opens the morning right after Castle and Beckett’s first night together. Castle wakes up to an empty bed and wonders if he had dreamed the whole event, when Kate walks in bearing coffee for them and wearing only his shirt. The two agree that they want to pursue a serious relationship and that their night together was not a one-time thing. The two are obviously reluctant to admit that they are finally an item when Castle stashes Beckett in his closet when Martha and Alexis show up unexpectedly and later when Beckett wants to do the same thing when Ryan shows up at Beckett’s apartment while Castle is there with her.
While their relationship was forged during the time when Beckett has resigned, the two are united by their determination to solve murder cases. This drives Beckett to ask for her job back from Gates. The two’s new romance at the 12th Precinct is complicated. NYPD ethics policies forbid romantic or sexual involvement between employees, and Beckett fears that Captain Gates will use the rule as an excuse to have Castle leave the precinct for good. So at work, the two officially decide to keep their relationship a secret. Castle and Beckett take a weekend getaway to the Hamptons in Murder, He Wrote, leaving Ryan and Esposito to attempt to discover Beckett’s secret boyfriend’s identity. Ryan finally succeeds when interrogating a suspect who had run into Castle and Beckett in the Hamptons. However, he decides to keep the secret as well until Castle becomes a murder suspect in Probable Cause and Beckett’s objectivity to evidence incriminating him becomes an obvious issue. Even then, he decides to only tell Esposito, and the two decide to keep Beckett and Castle’s relationship a secret as well unless they absolutely must reveal it.
Beckett’s commitment is tested in The Squab and the Quail when she has to spend the night in a hotel room with billionaire Eric Vaughn. Vaughn asks her if she’s with Castle, and she replies with a yes. But when Vaughn asks her if the relationship is serious, she hesitates. Vaughn leans in for a kiss, and just as Beckett starts to push away, they are interrupted by two bullets that pierce through the window. Luckily, no one got hurt, except maybe Castle when Ryan asks Beckett where she was standing when the shots were fired. She shows him, along with where Vaughn was, but Castle points out if he had been standing there, the bullet would have gone straight through his head. Beckett blurts out that he kissed her, and after a few moments of doubt, Castle leaves it alone.
In the episode Still, Beckett steps on a floor plate, setting the trigger for a bomb. To keep Beckett’s mind off the predicament she’s in, Castle distracts her with the argument of who was into whom first. As it’s discovered the bomb also has a timer and will detonate soon, Beckett asks Castle to leave. As he’s leaving, Becket calls out and tells him that she loves him, and he tells her that he loves her too. This is the first time we hear Beckett say out loud that she loves Castle. Castle shortly returns, and with the help of Ryan & Esposito, he and Beckett figure out what the disarm code is for the bomb, and Beckett is saved. As Castle and Beckett exit the apartment to the street, the two embrace, and just as they’re about to kiss, a car pulls up with Ryan, Esposito and Captain Gates, who rush up to them to show they’re happy they made it out alive. Gates says she’s glad that they’re both ok, and she “didn’t know too many men that would have done what he did” to stay with Beckett. As Beckett agrees, Gates tells Beckett, “Oh for heaven’s sake detective, just kiss the man. What, do you think I’m an idiot? I needed to maintain plausible deniability, which I can continue, as long as you two both act professionally at the precinct.” So their relationship is now out in the open.
In the season 5 finale, their relationship undergoes major decision-making. Beckett has been offered a big time job in Washington DC, but fails to tell Castle about her grand opportunity. When Castle eventually learns about it himself, he confronts her about not telling him, and they both have a fight. They ask themselves and their parents later on: what do they want with their relationship? Beckett eventually, under advice from her father, decides to take the job since it is what she knows she wants. Her decision is also made because she remains unsure of what she and Castle have is truly real or not, where the relationship is ultimately heading and if all they fell in love with is just “the dance” the two have been in and fears what will happen “when the music stops”. She calls Castle and tells him they need to talk, to which he also agrees. When the two meet up, she tells him she is taking the job. Castle begins talking about their relationship and how they have never truly defined it and Beckett thinks that they are going to break up. Instead, Castle gets on one knee and displays a ring and proposes. Beckett is left shocked in silence as she simply looks at him.
Valkyrie picks up where the finale left off. After an uneasy awkwardness, Beckett admits she got the job in D.C and admits to Castle she does love him, but she doesn’t want to miss her shot at the job. However, Castle reaffirms her by saying he was proposing because he couldn’t imagine his life without her, not to keep her in the city. With a smile, she says, “Well then, Richard Edgar Alexander Rodgers Castle, yes. I will marry you.”
In Need To Know, Castle admits that the distance between them is too hard for him and proposed a solution: an apartment in D.C. for him and Beckett. This shows Castle is willing to compromise for Beckett as she was both shocked and delighted by this gesture. However, everything came crashing down when her new partner Rachel McCord showed up and told Beckett she was fired due to tipping off the press in an investigation.
Beckett and Castle are seen trying to pick out a date for their wedding in Deep Cover, and by the end of the episode, Castle decided to cancel his book tour to have their wedding in September.
In Limelight“, Beckett is uneasy about the rumour that Castle and his ex-wife Gina are getting back together. Castle tells her that since she doesn’t want their engagement to be announced, those kind of rumours are bound to happen. Later in the episode Beckett decides to announce their engagement to a newspaper to quell those rumours, claiming that she needs to accept his celebrity status, and if there is news about Castle then she wanted it to be the truth; about them.
The season six finale shows Beckett and Castle heading to get their marriage license. However, once they get there it surfaces that Beckett is already to married to Rogan O’Leary. The two got married when a much younger Beckett went on a trip with him to Vegas. Castle and Beckett spend the episode doing a favor for Rogan in order to get him to sign the papers. After he signs, the two head to the Hamptons for their wedding.
On the way to the Hamptons, Castle’s car is driven off the road and the season ends with Beckett looking down at his burning car.
Season 7 opens with Beckett continuing the search for Castle. Three months pass before he is found in a dinghy at sea. The next few episodes are spent with the two trying to return to normal and deciding to put off the wedding expectations for a while.
Castle and Beckett finally head back to the Hamptons and tie the knot at the end of Time of Our Lives, in the presence of his mother and daughter and her father. In Once Upon a Time in the West, they visit an Arizona dude ranch under the pretense of an undercover honeymoon while investigating a murder.
Beckett once dated FBI Agent Will Sorenson for 6 months after they met and worked together on a kidnapping case of a 6-year-old boy. In this case, they found the kidnapper, but the boy was already dead when they got to him. When Sorenson got a career opportunity in Boston, he chose to took it, although he seemed to have asked Kate to come with him. She declined because coming with him would have meant moving away with him again if more opportunities rise in other places. Will missed Beckett, though, and meant to call several times but never got around to it.
In “Little Girl Lost“, Agent Will requests for Kate’s assistance in another kidnapping case, promising that they will find this child in time, which, eventually, they do.
Obviously, Will used the case as an excuse to get close to her again, although he was not expecting the presence of a shadowing novelist. This becomes evident when Will kisses Beckett when he gets her alone, although their moment is disrupted when Castle walks in on them to pop a theory.
Later, he asks her for another chance. When Beckett asks him what they will do if he has to leave again, Sorenson says that she should come with him then. When Beckett does not respond, Sorenson simply asks Beckett to think about it. Richard tells her that he could see why their relationship wouldn’t work, saying that they are too alike, like “yin-yin”.
In “A Death in the Family“, Sorenson is in the car driving former mobster Jimmy Moran back to the safe house, when the killer pulls up next to them and shoots both Moran and Sorenson. Beckett later looks genuinely concerned at the news of his injury, and waits at the hospital with Sorenson until she has to leave for work purposes. She returns after closing the case, laughing and enjoying her conversation with her ex-boyfriend.
It is presumed that he was dispatched to another location because he was never again seen since, although in “Murder, He Wrote“, Esposito finds out that he has been stationed in Europe six months prior.
In “Den of Thieves“, Beckett meets robbery detective Tom Demming, whom she instantly takes a liking. It is later revealed that the two have started dating, eating together at work, and even “sharing a cab.” At the end of “Overkill,” she shares a kiss with him, confirming that they are now in a relationship. However, in the season 2 finale, Beckett realises she has feelings for Castle, and decides to accept his invitation to the Hamptons even though she’d previously declined it. Beckett breaks off the relationship with Demming, claiming that it “isn’t what she’s looking for right now”. Demming asks her what she is looking for, and she merely looks away.
In the last scene of season 3’s fourth episode, “Punked“, Josh (Victor Webster) comes to the precinct and picks Beckett up to go motorbike riding (shown by her going to get her helmet). Prior to this, Beckett had not told anyone that she had a new boyfriend. In the next episode, it is revealed that Josh is a biker and a cardiac surgeon.
In “Last Call“, Beckett tells Castle that she and Josh had just been tucked up in bed with her laptop when his call came through to meet him at “The Old Haunt” and in “Knockdown“, Castle arrived at Beckett’s apartment and asked for Josh before he handed her flowers, indicating that the relationship with Josh is no longer casual.
In “Countdown“, Josh stays in Manhattan, as opposed to going to Haiti, causing Beckett to think they “have a chance.” Josh saves both her and Castle from hypothermia. They embrace at the end of the episode, however, Beckett seems distracted and longingly stares after Castle as he leaves.
At the end of “Rise“, in which the couple are seen together several times, Beckett reveals that she broke up with Josh during her recovery.