Cast

Nathan Fillion as Richard Edgar Castle/Richard Alexander Rodgers, a best-selling mystery writer. Plagued with writer's block after killing off his popular lead character, Derrick Storm, whom he no longer found inspiring, Castle found himself involved in the case of a copy-cat killer recreating murder scenes from his novels. He discovers a new source of inspiration in Kate Beckett, the detective leading the investigation, so uses his friendship with the mayor of New York to become involved in her investigations as a civilian consultant. He regularly uses his knowledge and talent to help solve murders, usually by taking evidence and assembling it into a narrative to explain their current case. He is twice-divorced and shares an apartment with his daughter and his mother. Earlier seasons of the show depict him as an immature womanizer; however, later seasons show a more mature and sensible side of him albeit only slightly. His close working relationship with Beckett results in him falling in love with her. He proposes at the end of the fifth season, and the two become engaged.

Stana Katic as Detective Katherine Houghton "Kate" Beckett, a homicide detective with the NYPD's 12th Precinct. Beckett is a first-class investigator who has gained a reputation for being intrigued by unusual cases. She joined the force after her mother was murdered in an unsolved case.  After meeting Beckett, Castle is inspired to begin a new series of novels about a female NYPD detective named Nikki Heat, clearly based on Beckett. Initially cold and hard towards Castle, she soon warms up to him and eventually falls in love with him. In the fourth-season finale, Beckett resigns from the NYPD after she and Det. Esposito hide vital information regarding a case from Captain Gates, but later reneges on this and is allowed to return after she serves her suspension. She also briefly worked as a Special Investigator for the Attorney General before returning to the NYPD. In the first episodes of season five, it became clear that Kate and Castle are in an intimate relationship, and the two become engaged at the start of the sixth season.

Jon Huertas as Detective Javier "Javi"/"Espo" Esposito, a former Army Special Forces soldier who now works in the homicide division 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 recognizes Castle and Beckett's attraction from the start and sometimes advises Beckett on her relation to Castle. He and Kevin Ryan are close friends, even though they constantly argue about obscure facts.He had a brief relationship with Dr. Lanie Parish in the third season, which ended with things being awkward between them making them unable to work around each other for some time. He was suspended from the NYPD for hiding vital information from Captain Gates in a case concerning Beckett's well-being. In that case, he felt betrayed by his partner, Det. Kevin Ryan. He has since been reinstated on the force and has reconciled with Ryan.

Seamus Dever as Detective Kevin Ryan, a former Narcotics detective who also works as part of Beckett's team. He and Esposito regularly tease Beckett, but they are also protective of her.  The character was a late addition to the show as the original presentation pilot was only 37 minutes long.  Ryan marries his girlfriend, Jenny O'Malley, in season four. In the final episode of season four, Ryan fails to support Beckett and his partner Esposito, which jeopardizes his partnership with the two of them though they reconcile later. In season five he is revealed to have worked undercover in the Irish mob. In season six, Jenny gives birth to their daughter.

Tamala Jones as Dr. Lanie Parish, a medical examiner with an upbeat outlook. She is a friend of Beckett and is one of the few people Beckett can talk to easily and without reservation. Parish notices the personal connection between Beckett and Castle and urges her to act on it. In the third season, Lanie and Esposito began a relationship they believed was secret, though the other main characters were aware of it. They later broke up, making it awkward for them to work together for a time, but have since reconciled.

Ruben Santiago-Hudson as Captain Roy Montgomery (Season 1–3), Beckett's boss and Captain of the 12th Precinct. Montgomery appreciates Beckett's determination and diligence but maintains a close watch to make sure the investigations go smoothly. He is amused by how much Castle annoys Beckett but also appreciates how effectively the two work together. At the end of the third season, he is killed after staging a trap to capture a hired killer involved in the conspiracy that killed Beckett's mother.

Molly Quinn as Alexis Castle, Castle's teenage daughter by his first wife Meredith. Alexis is unusually intelligent for her age, and is often more mature and responsible than her father. She is very bright and enjoys school. A running gag in the series has her begging Castle to force her to study and do her homework instead of "fun" activities like parties and overnight trips. Her social and interpersonal issues often parallel some aspect of the case Castle and Beckett are investigating, and Castle sometimes asks her opinion on a killer's motivation.

Susan Sullivan as Martha Rodgers, an actress on and off Broadway and Castle's mother. She lives with him and Alexis after an ex-husband ran off with her life savings. She helps her son through the difficulties of raising a teenage girl (even one as mature as Alexis) but also has an active social life of her own. She is free-spirited and often gives Castle valuable advice regarding Beckett.

Penny Johnson Jerald as Captain Victoria Gates (Season 4–), Captain Montgomery's replacement. Formerly with Internal Affairs, Captain Gates (referred to by some as "Iron Gates") is more interested in career advancement, and takes a dimmer view than her predecessor of the Beckett-Castle dynamic. Strict and unforgiving, she loathes Castle's presence at the precinct regarding him as an unprofessional, distracting and juvenile presence who interferes with the way she would prefer to run things. As the series progresses she comes to tolerate Castle, and even grudgingly accept that he is useful for solving cases. Though her outright hostility fades, she is nonetheless strictly by the book, and still willing to beat Castle with it when needed.