Kate Lockley - Angel
Kate first appears in the episode "Lonely Hearts" where she is undercover trying to catch someone who is murdering women from the singles bar. She is a young, skeptical detective for the L.A. Police Department. She appears in 16 episodes in season 1 and 2. She often finds herself in over her head, but thanks to Angel's help she manages to get through it. After Kate's father is killed by vampires she becomes obsessed with supernatural cases. This ultimately leads to her undoing. Her last couple appearances of the show involve her being fired and then overdosing on pills. Angel saves her and they share a touching moment before she disappears from the show for good.
Serena Southerlyn - Law & Order
Serena comes in to replace Abbie Carmichael as ADA under Jack McCoy. She is known to be a supporter of gay rights, and advocates for defendant who's crimes are a byproduct of social circumstances. She apposes the death penalty and frequently disagrees with Branch's conservative opinions. She leaves the show after 85 episodes with what is considered one of the biggest water cooler moments, when she asks Branch if he is firing her because she is gay (something that had never been mentioned before).
Lauren Gilmore - Heroes
Lauren first appears in Season 4 as a co-worker of Noah's. We learn that they almost had an affair years ago before Noah called it off. Lauren then has the Haitian wipe her memory. Noah then runs into her at the grocery store and they rekindle their relationship. Lauren works for the CIA and helps Noah find the Carnival and take care of Samuel. She is shot in the process, but suffers only a minor injury. She is at the Carnival with Noah when Claire exposes her special power to the world in the final episode of the series.