Javier Esposito (Jon Huertas) is a bilingual (English and Spanish) detective who works in the homicide division with his partner, Kevin Ryan, as part of Beckett's team. He is always ready with a sardonic comment to keep Beckett on her toes and enjoys the way Castle riles her. He and Ryan are best friends, even though they constantly argue about obscure facts. He was disappointed when Ryan felt obliged to ask his fiancée's brother to be his best man instead of him. It is implied in the pilot "Flowers for Your Grave" he is a fan of Shark Week, and in the season two episode "Den of Thieves", it is revealed that Esposito transferred out of the 54th Precinct and has been partnered with Ryan for three years prior to the pilot episode. In season three's "Poof, You're Dead", he and medical examiner Lanie Parish began a relationship that they believed was a secret. However, their relationship was Common knowledge to the other characters, who were playing along out of respect for their privacy, and in the episode "Lucky Stiff", Ryan tells Esposito that everyone knows about the relationship. In the season four episode "Demons", Esposito reveals that he and Lanie are "taking a break" after an awkward incident on a double date. In "Almost Famous", it is implied that he was in the Green Berets; this point of Esposito's personal history is developed in subsequent episodes "Knockdown" and the two-episode arc "Set-Up" and "Countdown". In the episode "Kill Shot" he displays technical knowledge of military sniping and refers to his having served in a sniper unit, implying that he received training.

With Bracken turning his attention to the Presidency, he had Maddox try to retrieve Montgomery's evidence before going after Beckett. After Maddox is killed trying to retrieve the files, Castle and Beckett are finally able to identify Bracken through the remains of Montgomery's payments to Bracken's bank account, the evidence Smith held for Montgomery. Now knowing the name of the man who had her mother killed, Beckett approaches Bracken at a political fundraiser and convinces him to leave her alone, or else his secrets will become public. Bracken appears again in "Recoil", when the 12th Precinct is able to connect a murder victim directly to him, only to discover that Bracken has become the target of a political Assassin as he prepares his Presidential campaign. Beckett is forced to help save his life by thwarting the Assassin, and Bracken acknowledges that he owes her his life before approaching the press and uses the circumstances behind the episode to present himself as a Politician willing to stand up against corrupt senators, as the Assassin was hired by his former mentors, and consolidate public perception of him as a Presidential candidate.

Captain Victoria Gates (Penny Johnson Jerald) (seasons 4–7) is Montgomery's replacement as captain of the 12th Precinct. At her own insistence, she is called "Sir" by the officers who report to her. Formerly with Internal Affairs, Gates, known to some as "Iron Gates", is more interested in career advancement and the reputation of the precinct, and by extension, the NYPD, than in the finer points of interpersonal dynamics, making decisions based on the political impact that they would have. This contrasts with the style of her predecessor, who made decisions based on how they would affect people (for Example, concealing a policewoman's vigilante activities because of the potential political backlash), and takes a dimmer view than her predecessor of the Beckett-Castle working relationship. Although she is obliged by the mayor to allow Castle to stay on in his consulting role, Gates tries to convince Beckett that she can be a better detective without him around. In the episode "Still", Gates reveals that she was aware that Castle and Beckett were in a relationship, but tells them that she did not say anything in order to maintain her own plausible deniability, and she promises to continue her silence on the condition that the two be professional on the job. In "Dial M for Mayor" she encourages Beckett to perform her duties objectively and dispassionately, stating her attitude in IA and Homicide was not because she hated police officers, she works hard to hold people accountable so NYPD officers like her father, uncle, and subordinates can bear the cost of serving the people of New York free of corruption. She has a sister, named Elizabeth Weston, who's a US Attorney. The two have a tense relationship as years before, Gates had to bust a corrupt cop involved in one of Elizabeth's investigations despite Elizabeth's protests. They reunite over another case and put aside some of that bad feeling.

Bracken plays a pivotal role in the season six episode "In the Belly of the Beast", when Beckett is recruited to a risky undercover operation to infiltrate a drug ring and investigate a connection between the ring and the Russian mafia in a bid to identify the kingpin, known only as Lazarus. Beckett discovers that the proceeds from the drug ring are being laundered through the Future Fund, a political fundraiser collecting money to support Bracken's Presidential bid. She also learns that Bracken had ordered one of his contract killers to save her after Vulcan Simmons—the figurehead for the ring and one of the original suspects in the murder of Johanna Beckett—blows her cover, as a means of repaying his debt to her. Beckett predicts that the next time they meet, all bets are off.

On May 29, 2015, Penny Johnson Jerald revealed through her Twitter account that she is no longer a cast member of Castle. She stated that she was surprised and saddened by this occurrence. Her character was written out by promoting Gates to Deputy Chief and reassigning her to One Police Plaza.