While The Rookie started out focused on Nathan Fillion's John Nolan, it has expanded considerably. The ABC series has steadily introduced new people around Nolan and developed their characters to make itself feel like more of an ensemble show. Some of those characters have also helped Nolan continue to grow.
In Season 3, The Rookie introduced Jenna Dewan's Bailey Nune as a love interest for Nolan. By the next season, Dewan was promoted to series regular as Bailey enabled Nolan to get out of his white-knight-esque rut. Now as Season 5 approaches, the series is making a major alteration to its premise by exploring Nolan's future as a training officer. That requires another new character -- Celina, played by Lisseth Chavez.
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According to TVLine, "Celina has an easy confidence and high marks at the Academy, and her unconventional approach to police work will pose an unexpected challenge for her training officer." While that description is noticeably vague, the assumption is that Celina will be Nolan's trainee. Late in Season 4, Nolan made it clear that he wanted to become a training officer, and at San Diego Comic-Con 2022, Fillion confirmed that his character would take that step during Season 5.
With that said, The Rookie is doing something kind of odd with the Celina character. Chavez (who most recently recurred on DC's Legends of Tomorrow before it was canceled on a cliffhanger) has been tabbed as a guest star for multiple episodes. However, she is not a regular in Season 5, meaning that while Nolan will be in every episode, Celina will not.
Some fans would point to The Rookie's previous casting tendencies and say that Chavez starting out as a guest star is normal. Aside from the aforementioned Jenna Dewan as Bailey Nune, Shawn Ashmore's Wesley Evers was a recurring character in Seasons 1 and 2 before being upped to regular in Season 3. Likewise, Tru Valentino's Aaron Thorsen was recurring in Season 4 but was just promoted for Season 5. However, it's a bit strange to do that with Chavez if Nolan being a T.O. is the focus of his season.
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There's a faction of the Rookie fandom that thinks that Chavez being cast as a guest star is indicative of a lack of confidence in the training officer storyline. That's because she previously played Vanessa Rojas in Chicago P.D. on a similar recurring-to-regular basis in Season 7, but her character disappeared before Season 8 without any explanation. Chavez was let go by producers and Rojas' fate is still unknown as Chicago P.D. heads into Season 10.
With that history in mind, it kind of sounds like The Rookie isn't fully committed to making Chavez part of its universe, either. It's entirely possible that the series is easing Celina into the action, but it seems like she should be a regular participant given the direction that Nolan is taking. Starting Celina as a recurring character gives the writers a chance to similarly have her leave if she doesn't fit in, and allows for the possibility that Nolan being a training officer is a one-season (or less) experiment.
The Rookie Season 5 premieres Sept. 25 at 10:00 p.m. on ABC.