Susanna is Michael Cordero's new partner at the Miami Police Department. Barnett comes across as a very cunning and knowledgeable Detective. She is skeptical of Michael's motives and actions, but neither to the detriment of her professional nor emotional judgement of him. She is later revealed to have been investigating Cordero for Internal Affairs.
When she was 5 years old, her father left her and her mother, and she has never seen him since. Susanna is allegedly from Tuscaloosa, Alabama, but is revealed to be Sin Rostro in disguise.
Barnett is hired as Michael Cordero's new partner, following the departure of Nadine Hanson, but is really tasked by their superior, Lieutenant Armstrong, to suss out whether Michael is working with Nadine or had a hand in Nadine's tracker being disabled at the time of Mateo's kidnapping and safe return.
Following Michael's break-in at the station to check the security tapes, Barnett is the one to negotiate with Armstrong that Cordero be given a faux public dismissal to allow him to work undercover. Once Michael returns with information pointing them in the direction of Mutter, the two seem to work very well together and develop a bantering professional relationship.
Following the realisation that Mia Alver is not Mutter, Susanna breaks the news to Luisa, who kisses her and Susanna responds, but leaves abruptly. After Michael Cordero reveals to Susanna that he let Nadine go in order to return Mateo to Jane, she writes up a report to give to their superior, but Michael convinces her to stall in exchange for his knowledge on the case.
Turns out Susanna and Luisa did start something, but Susanna ends it when Luisa acts as a mole in the investigation and professes her feelings for Rose. After Luisa agrees to bait Rose by 'being in a car accident and sent to the hospital', Susanna is the detective on call to capture Rose, but Rose threatens her sister and nephew, and holds Susanna hostage to leave the hospital, shooting Susanna in the process.
Susanna survives with her arm in a sling, but is determined to suss out why Mutter killed Rose and where Mutter is, in order to protect her family. She puts her budding romance with Luisa on hold, after Luisa falls off the wagon following Rose's death and convinces Luisa to go back to rehab. Susanna tells her she can take Luisa out when she gets better. It turns out all Susanna has said and been was a lie, as there is no Susanna Barnett. It's really Rose, disguised as Susanna.
- Luisa has a crush on Susanna and Susanna has feelings for Luisa, almost from the moment they meet, which makes sense considering she's really Sin Rostro.
- Luisa kissed Susanna in Chapter Thirty-One and Susanna responded briefly, before breaking off the kiss and leaving.
- Susanna is given a made-up mother and brother.
- She and Michael are on good terms after she revealed that she was investigating him for IA.