Nothing is known about Sierra's life from before the other than she lived in Los Angeles.
Sierra was one of the millions of people living in Los Angeles when the outbreak ravaged the city. At some point, she was able to flee the city before Operation Cobalt destroyed Los Angeles. Afterwards, she started her journey south towards the Mexico-United States border where she wound up at a military camp.
Sierra is seen cautiously walking around a seemingly-abandoned military camp, navigating around a dead soldier slumped up against a truck and smoldering ruins of tents. She spots an injured woman running through the camp when suddenly she is attacked by two zombified soldiers. However, Sierra is able to put down both soldiers with her knife.
Sierra, aware of Gabi, tricks her after removing her hazmat suit and placing it on a walker. Sierra surprises Gabi and demands to know why she is following her. Gabi admits it is because she is injured and was using her to kill nearby walkers. Sierra then uses her as a shield to dispatch the walker wearing the hazmat suit before walking away. Gabi pleads with her not to go.
Sierra climbs into a guard tower with Gabi behind her and wipes her bloody knife on her suit pants. Gabi suggests they head south across the border into Mexico, claiming that it should have been overwhelmed with sick people trying to cross the border, allowing them to slip past it. Gabi also mentions that her boyfriend was with the Border Patrol and knows of a tunnel, but Sierra interrupts and suggests that her boyfriend is probably dead because he hasn't come back yet. While Gabi offers to take Sierra because she needs a place to go, Sierra reaffirms that she doesn't need Gabi to survive.
Sierra walks through the desert with Gabi behind her, having left the military camp. As the women approach the Mexico-United States Border, Sierra informs Gabi that she was from Los Angeles but escaped when the bombs fell and indicated that she may have lost someone in Operation Cobalt. After learning that Gabi lost a sister, she teaches her how to kill a zombie, advising her to aim for the eye socket so the knife does not bounce off the skull when stabbing it.
Gabi and Sierra enter a fenced-in construction site when suddenly Gabi is attacked by a zombified construction worker on a chain. Sierra yanks the zombie away and gives Gabi her knife so she may practice defending herself. Before she can put the zombie down, a gunman shoots the zombie, splattering blood on Gabi's face.
Sierra watches on as Gabi is reunited with her boyfriend, Colton. She remains silent throughout the reunion, receiving Colton's thanks for saving Gabi with a curt nod. As Gabi and Colton embrace, she picks up her knife.
Sierra watches from behind as Colton and Gabi embrace. After Gabi enters the tunnel, Colton tells Sierra that he can't let her come down or let anyone else know about the tunnel. She pleads with him that she just wants to escape to a safe place and proceeds to subdue him after he raises his gun at her. While disarmed, Sierra stabs Colton in the leg and quickly enters the tunnel.
Sierra descends into the tunnel and informs Gabi that Colton tried to kill her and that he had abandoned Gabi. She tells Gabi they have to leave, ignoring Colton's cries from above that the tunnel is unsafe. She jogs down the tunnel, where she hits a trip wire.
The Tripwire triggers a bomb which causes and explosion, making Gabi and Sierra fall to the ground. Sierra calls out for Gabi, telling her to follow her voice meanwhile her ears are ringing and she tries to activate a flashlight.
Sierra limps through the tunnel, hearing Colton and Gabi looking for her. She frantically looks for a place to hide, finally finding a mining cart. Once inside the cart, she is attacked by a walker.
Sierra is limping through the tunnel and hears Colton and Gabi talking behind her. She frantically searches for a place to hide, ending up in a mining cart, where she is attacked by an Infected.
Gabi uses the mining cart to sneak up on the Unnamed Border Patrol Guard where she shoves an infected into him and then hits the Second Unnamed Border Patrol Guard with a large stone. She is about to make an escape up a ladder when she hears Gabi calling for her.
After responding to Gabi's call, Sierra realizes that Colton has Gabi at gunpoint. She sneaks behind him and uses a fabric in an attempt to strangle him. Meanwhile Colton reaches for his gun, Gabi takes the opportunity to grab a knife and stab Colton in the stomach. Sierra gets up and stands beside Gabi, not faulting her by making her understand that she had no other choice. Immediately, they both flee.
This is a list of victims that Sierra has killed:
- Numerous counts of zombies.
After realizing that she was being followed by Gabi, Sierra was cautious when she confronted her and threatened her with a knife. After a couple of pleads Sierra agreed to keep her safe for the time being. When Gabi mentioned a safe place, Sierra was somewhat skeptical but agreed to help her find it. Meanwhile they traveled together, Sierra was informed that Gabi lost her sister to the infection and that she didn't know how to end it. To show compassion, Sierra took it upon herself to instruct her on the proper way to kill the undead.
When meeting Colton, he attempted to shoot Sierra. Despite Gabi's relationship with Colton, Sierra was prompted to defend herself and escape; and although she completed her part of the bargain, Sierra showed signs of wanting Gabi to continue following her.
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