Season one

In the series' beginning, LaGuerta is depicted as a determined and ambitious woman with a stronger aptitude for political maneuvering than practical police duties. She holds a deep animosity towards Debra, Dexter Morgan's sister,^[2] but harbors an attraction to Dexter. In the conclusion of the first season, Captain Tom Matthews, her superior, removes her from her leadership position due to her inability to apprehend the Ice Truck Killer.^[3]

Season two

In the early episodes of season two, LaGuerta grapples with her demotion and dissatisfaction with her replacement, Esmee Pascal. To retaliate, she engages in a romantic relationship with Pascal's fiancé, Bertrand, which fuels Pascal's growing paranoia and instability. As Pascal's behavior becomes increasingly erratic, Captain Matthews is compelled to remove her from her position and reinstate LaGuerta.

When her friend, former partner, and former lover, James Doakes, becomes the primary suspect in the Bay Harbor Butcher case, LaGuerta endeavors to vindicate him. She receives information from a dubious source regarding two Special Forces missions undertaken by Doakes, which coincide with the deaths of two Butcher victims. In the season finale, Doakes and drug dealer Jose Garza are discovered dead in an apparent accidental explosion, leading to the case's closure. Despite the evidence LaGuerta uncovers, it is disregarded. Following Doakes' demise, LaGuerta appeals to police officers for contributions to his memorial fund, refusing to accept the notion that her deceased friend was a serial killer.^[4]

Season three

In season three, following the aftermath of Doakes' demise, LaGuerta takes charge of an investigation involving the murder of Oscar Prado, the brother of her former boyfriend, Assistant District Attorney Miguel Prado. Additionally, she develops a close bond with defense attorney Ellen Wolf and experiences deep sorrow upon learning of Wolf's murder. Through her own personal investigation and input from Dexter, she uncovers that Miguel is responsible for Wolf's death. LaGuerta confides in Dexter, considering him the only other person aware of the truth. After Miguel's demise at Dexter's hands, LaGuerta becomes distressed by the possibility of the Cuban community honoring Miguel by naming a highway after him. She desires to obtain solid evidence to prove Miguel's guilt in Wolf's murder. However, Dexter manages to persuade her that pursuing the case would only inflict further pain upon Miguel's family and the community, and even if successful, the evidence would be insufficient. Reluctantly, LaGuerta lets go of the matter.

Season four

In the beginning of season four, it is revealed that LaGuerta is involved in a romantic relationship with Detective Angel Batista. They choose to keep their relationship hidden from their colleagues at the homicide department, except for Dexter, whom both Batista and LaGuerta confide in. However, their situation becomes complicated when LaGuerta discloses their relationship to their superiors to prevent it from being used against them in a trial. As a consequence, they face the possibility of being reassigned, requiring one of them to leave the homicide department. Recognizing the significance of their jobs for their identities, Batista and LaGuerta decide to end their relationship and provide signed affidavits to confirm this. Despite their intentions, staying apart proves challenging, and they resume their secret encounters. To avoid potential consequences from higher authorities, they clandestinely marry, with Dexter serving as a witness.

Season five

In season five, LaGuerta and Batista experience difficulties in their relationship. LaGuerta becomes increasingly demanding of Batista, while he starts displaying more impulsive behavior. Their problems escalate when LaGuerta publicly blames Debra for a mistake that she herself made, resulting in the loss of multiple lives. This incident further strains Batista's perception of LaGuerta. However, by the conclusion of the season, they had reconciled and mend their relationship.

Season six

LaGuerta and Batista go through a divorce after realizing their divergent desires. Following her blackmailing of Matthews with a contact list belonging to a madam, which includes his name, LaGuerta is promoted to the position of captain. Matthews assigns Debra as the new lieutenant, a decision that upsets LaGuerta, as she had promised the role to Batista. Initially uncertain, Debra seeks advice from LaGuerta, who intentionally provides her with unsound guidance in an attempt to undermine her. However, Debra gains confidence in her own abilities and becomes self-reliant. When evidence of Matthews' involvement with another prostitute emerges, LaGuerta initially instructs Debra to conceal it. However, LaGuerta warns Debra that she will face consequences if she deviates from the expected course of action.

Season seven

At the start of the season, LaGuerta discovers a blood slide containing the blood of serial killer Travis Marshall at the scene of his death. Recognizing similarities to the Bay Harbor butcher's methods, she begins an investigation to determine if the butcher is still alive and seeks to prove Doakes' innocence. Her suspicions turn toward Dexter when she learns about his boat's relocation during the Butcher investigation. LaGuerta seeks assistance from Matthews, who assures her that her suspicions are unfounded.

As LaGuerta and Matthews delve into the case, they revisit the cabin where Doakes died and uncover evidence linking Santos Jimenez to the deaths of Dexter and Brian Moser's mother. LaGuerta's suspicions of Dexter escalate after learning about his background and the disappearance of Jimenez. They search the area and discover evidence implicating Doakes, which LaGuerta believes was planted.

Eventually, LaGuerta devises a plan to confirm Dexter's identity as the butcher. She orchestrates the release of Hector Estrada, the man who ordered the killing of Dexter's mother, hoping that Dexter will target him. Dexter indeed tracks down Estrada but realizes LaGuerta's setup. Reluctantly, he releases Estrada to ensure his own escape, though LaGuerta deduces his intentions upon discovering the kill room.

LaGuerta has Dexter arrested for Estrada's murder, but forensic specialist Vince Masuka provides evidence that proves Dexter's innocence, leading to his release. Despite this setback, LaGuerta remains determined to apprehend Dexter, especially after finding circumstantial evidence linking him and Debra to the scene of Marshall's death. She confronts Debra with the evidence, but Debra defends herself.

As tensions escalate, Dexter concludes that the only solution is to kill LaGuerta. He abducts and kills Estrada, then renders LaGuerta unconscious upon her arrival. His plan is to stage the scene to make it appear as if LaGuerta and Estrada killed each other. However, Debra interrupts him and pleads for LaGuerta's life. LaGuerta implores Debra to kill Dexter, asserting her own moral superiority. Dexter relents and drops his weapon. Tearfully, Debra shoots a surprised LaGuerta in the chest, resulting in her death.^[5]