Kate Lockley is a fictional character created by Joss Whedon for Angel. She is portrayed by Elisabeth Röhm. Lockley first appears in the episode "Lonely Heart" as a young, skeptical detective for the Los Angeles Police Department. Gradually, she becomes more hardened as she learns of the supernatural world.

Kate Lockley is a young police detective working for the LAPD, initially unaware of the supernatural. She first appears in "Lonely Heart", investigating a case involving the brutal murders of several patrons of D'Oblique, a popular singles bar. After encountering Angel as he leaves the site of another murder, she instantly suspects him to be the killer, unaware that Angel had actually been attempting to stop the real killer. Angel escapes, and Kate becomes obsessed with finding him to the point that she searches the Angel Investigations offices without a warrant. After she is attacked by the real killer and saved by Angel, she helps him to bring the killer down. Kate subsequently clears Angel of all charges, and Angel offers his services to her whenever she needs them.

Kate's subsequent encounters with Angel lead her to trust him as a helpful stranger, and she in turn, assists him, using her position on the police force to provide him with information.

Kate was raised by her widowed father, Trevor, who is also a police officer and taught her to put her emotions aside and to take care of herself at all costs. Through her father, she develops a complete devotion to her occupation, later on saying that being a cop is all she's ever known and that she wouldn't know what else to do. When she discovers that Angel is really a vampire and subsequently discovers his past as Angelus before his soul is restored, her black and white view of the world is shaken, and she delves into the darker side of Los Angeles. On one such investigation, Kate finds her father's lifeless body—completely drained of blood by vampires.

After her father's death, Kate's relationship with Angel becomes strained and eventually adversarial. She becomes obsessed with ridding the city of vampires and other demons. Kate is the first to investigate any calls matching the "otherworldly" profile, even if it is out of her jurisdiction. She has an intense dislike for Wolfram & Hart and the clients the law firm represents.

When a resurrected Darla frames Angel for murder and goes on a killing spree with Drusilla, Kate shows up as his arresting officer and lets him go as she believes he is the only one who can stop them. Although Angel catches them, he allows them to murder 13 lawyers, which causes Kate to distrust him again after she figures out that he not only let them murder but facilitated them by locking the doors.

Her obsession with supernatural cases becomes her own undoing: after Angel attacks a police captain who had brought back deceased police officers as zombies in an attempt to cut down on crime, along with her increasingly erratic and obsessive behavior, she is fired from the LAPD. Kate calls Angel and blames him for ruining her life, then attempts suicide by taking prescription pills with hard liquor. Angel hears her call and later goes to Kate's apartment. When she does not respond to his energetic knocks, he breaks down her door, finds her lifeless body on the floor, picks her up and holds her under a cold shower to resuscitate her. After her life is saved, she thanks him and then tells him to leave. That evening, Kate and Angel talk and she forgives him, believing there is a higher power at work and they are not alone in the never-ending battle against the forces of darkness. When Angel asks her why, she replies that she never invited him into her apartment, yet he was able to break in and save her.

She never appears on the series again, due to Elisabeth Röhm joining the cast of Law & Order. However, she is mentioned again in "Dead End" soon after her last appearance-Angel mentions to his friends that they no longer have a friend in the LAPD.