See Also: Notable Characters Season 1 | Notable Characters Season 2 | Notable Characters Season 3 | Notable Characters Season 4 | Notable Characters Season 5 | Notable Characters Season 6 | Notable Characters Season 7 | Notable Characters Season 8
- Hayley Shipton - Independent security specialist who comes to New York on the trail of identity thieves and ends up assisting Castle in his hunt for Beckett.
- Vikram Singh - Calls Beckett in a panic when 5 members of her former AG team are killed, and he thinks he's next. Vikram starts them on the trail of LokSat.
- Rita - Shows up in the nick of time to help Beckett and Vikram escape from Brooks' thugs; claims to be the wife of Jackson Hunt and to be working for some unnamed "three-letter agency".
- Father Arguello - Fakes blindness in order to get transferred out of Tampa, recruits a pathological liar to "adjust" city accounting records to cover his own sins, and then escapes Castle and Alexis on foot, revealing that he's not blind after all.
- Billy O'Rourke - Hit man from Boston who's given the task of offing Jimmy "Two Guns" O'Malley; he picks up Dave Johnson but realizes he's not O'Malley, so lets him go. Billy is having brunch with Castle when Ryan and Esposito confront him.
- Mia Laszlo - Professional "nose" (perfume analyst), called upon to identify a murderer, whom Castle enlists to help him gauge Beckett's feelings by her scent. Castle encourages Mia to get out and live life instead of hiding in her sterile apartment.
- The return of Ethan Slaughter, who turns out to have unexpected depth in the areas of culinary and musical arts.
- Caleb Brown - Public defender who shows up at the 12th on behalf of Salvador Acosta. Brown was previously an attorney at a corporate law firm; he drew up papers for an offshore shell corporation to establish ownership of the warehouse that Vikram and Kate believe housed LokSat's smuggled heroin.
- Caleb Brown returns.
- Rita returns.