Josh Davidson is Detective Kate Beckett's mysterious boyfriend in season 3. He is a cardiac surgeon who, like Beckett, rides a motorcycle.
He first appears in Punked (episode 3.4) when he drops by the precinct. Unlike with her last boyfriend, detective Tom Demming, this relationship takes not only Castle, but all of Beckett's colleagues at the precinct by surprise, since she never mentioned having a boyfriend prior to Josh's showing up at the precinct.
In Knockdown, episode 3.13, Beckett tells Castle Josh is in Africa "saving the world" and that he does not know about her homemade murder board of her mother's murder that she keeps hidden in her apartment.
In 3x16 (Setup), Castle refers to Josh as "Motorcycle Boy", then, when chastised by Beckett, revises the nickname to "Doctor Motorcycle Boy".
Beckett and Josh seemed on the verge of breaking up in Setup, the first part of a two-part storyline, when Castle enters the precinct one morning and finds them arguing at Beckett's desk. Later, Castle asks Beckett about him and she says when he saw them earlier, Josh was leaving to go to Haiti on another Doctors Without Borders mission. She confesses to Castle that she is frustrated by his frequent absences, although his dedication to helping the less fortunate was what initially attracted her to him and she pursued the relationship because it allowed her "to keep one foot out the door." Finally, she admits that what she really wants is a relationship where both involved can simply be there for each other and can "dive into it together." In the conclusion of the two-parter, Countdown, Josh is part of the team that rescues Castle and Beckett from freezing to death in an industrial sized freezer, and tells Castle he changed his mind about going to Haiti. For this act, Beckett forgives him and believes "we have a chance."
In Rise, Josh helps to save Beckett's life after she is shot in the chest, but is furious at Castle and blames him for her being shot and for Roy Montgomery being killed. They fight briefly, but are pulled apart by Beckett's father. Despite the fact that Beckett "really liked him," he and Beckett break up over the summer while she recovers because she realizes that she can't totally commit to a relationship until she solves her mother's murder.
He is played by actor Victor Webster.