The former Off The Map star has signed up to play Jaime, the younger sister of Angel (David Zayas) and Harrison's new nanny.
Garcia has now told TV Guide that Jaime has no idea about the dark side of Dexter (Michael C Hall).
"She's a very good nanny and absolutely adores Harrison and equally adores Dexter and thinks he's the best dad in the world," she said. "She's completely unaware of his extracurricular activities."
Garcia explained that Jaime and Dexter's "chemistry is great and their dynamic is interesting", but she admitted that she has no idea if the relationship could become romantic.
"I do know that they're in close quarters and she's basically the mother of his kid right now in raising him, changing his diapers and feeding him," she said.
"Harrison means the world to Dexter, so anyone who's helping Harrison live a healthy life in his developmental years obviously holds a good place in his heart. You only get one true love in every lifetime, so Lumen was probably that for Dexter."
However, Garcia admitted that she is not taking Jaime's safety for granted, saying: "When you're on a show with a serial killer, your head could roll at any time, which keeps everyone on their toes. Dexter leads a double life, and the underground life is dangerous, not only for him but for people close to him. She might be in danger - we'll see."
Other stars who have signed up to appear in the new season of Dexter include Jordana Spiro, Edward James Olmos, Colin Hanks, Mariana Klaveno, Brea Grant and Mos Def.
The show will return to Showtime on Sunday at 9/8c.
> Read more Dexter coverage
Watch a trailer for the new season of Dexter below: