The 'Final Girl' Trophy/Achievement in The Quarry requires that only Kaitlyn survives from the main cast. For this to happen, the player must kill off every other character. Failing QTEs and making specific choices can do this easily, but ensuring Kaitlyn survives is a little more complicated. This is how to unlock the 'Final Girl' Trophy/Achievement in The Quarry.
How to keep only Kaitlyn alive
When playing as Kaitlyn, they must do anything they can to keep her safe. Both Ryan and Laura must also survive long enough to kill Chris Hackett so that Max can be led to his death.
Getting the doll in Chapter One
During Chapter One, players must have Abi and Emma break into the cabin so Abi can examine the doll. Players must then keep the doll as it will be the only thing that can save Kaitlyn in Chapter Ten.
There are two ways that Jacob can die with the first method being to get Jacob shot by Ryan in Chapter Four. To do this, players must shoot into the trees after Ryan saves Nick and then be insistent. When given the option to shoot again, Ryan must wait out the timer until near the very end and fire into the trees again. If done right, this will kill Jacob.
The second method is to wait until Jacob runs away during Chapter Six. This requires players to fail the Don't Breathe event or to run until Jacob steps into the beartrap. Choosing to Pull Free and failing the QTEs will keep him there long enough to be killed.
When playing as Emma in Chapter Four, players can get her killed by opening the trapdoor instead of searching the bag. This will result in her dying instantly. Alternatively, if Emma escapes but is bitten, then players will be able to shoot Emma as Kaitlyn once she is freed from the car in Chapter Eight. She will be a werewolf and players can then choose to use silver shells to then kill her.
During Chapter Six, players will be able to get Abigail killed by simply not shooting Nick. When given the option to shoot at Nick, players can just let the timer run out and Abigail will be decapitated by Nick before running off. Abigail can be killed later, however, this requires that Emma survives, invalidating the requirements for 'Final Girl'.
Laura's death is dependent on her shooting Travis in Chapter Seven. When given the option to take Travis's gun, players must use it to escape and not apologize when leaving. This will cause Travis to kill Laura in Chapter Nine.
In Chapter Eight, while Laura and Ryan investigate the red room with cages, Laura will attempt to shoot the werewolf. Ryan has the option of interrupting but players should let Laura kill Nick.
There are two ways Dylan can die and both happen while accompanying Kaitlyn. When playing as Dylan during Chapter Nine, players will end up being chased in the scrapyard. When given the choice, Dylan must sound the horn and get the werewolf's attention. Failing all the following QTEs after will result in Dylan being devoured while Kaitlyn can escape.
The second method of killing Dylan is to keep him alive until Chapter Ten when finding a vantage point. Once the cabin is broken into, Kaitlyn can choose to run or wait. Choosing to wait will get Dylan killed as Kaitlyn shoots the werewolf and then runs away.
As long as players shot Travis in Chapter Seven, Laura will automatically be killed after Ryan kills Chris Hackett. The choice to bite Ryan won't matter as long as he is able to shoot Chris. If players fail to shoot Chris Hackett, then Max will stay a werewolf and wake up alive. This will then nullify the Trophy/Achievement.
Alternatively, players can have Laura, Ryan, and Travis work together until the end of the game if she did not shoot Travis in Chapter Seven. Choosing to not raise the gun will upset Travis, leading him to try taking the gun from Laura. Passing the QTE will have Travis trip, giving players the option to shoot Travis. This will alert Silas and if neither Laura nor Ryan have werewolf blood on their face, Silas will kill them both.
Ryan's death is dependent on Travis killing Laura. After she is stabbed to death by the silver mirror, players must choose to be aggressive, causing Ryan and Travis to fight for the gun. Failing the QTEs from here will cause Travis to kill Ryan in retaliation.
Alternatively, if Ryan didn't accept getting bitten, he will die of his knife wounds shortly after killing Chris. If Laura is alive, she can then die later in Chapter Ten after killing Travis but refusing to shoot Silas.
As long as Chris Hackett is killed, players will begin Chapter Ten playing as Max. When given the choice to stay or swim to shore, Max must swim to shore in order for him to be killed by a werewolf.
Keeping Kaitlyn Alive
In order for players to keep Kaitlyn alive, they must make sure to interact with the doll that is located in the kitchen during Chapter Ten. Without it, players will die no matter what choices they make. After interacting with the doll, players can continue like normal until the werewolf breaks in.
Players can then run or hide and Kaitlyn will eventually make her way to the kitchen. When given the choice to either run or go to the freezer, she must open the freezer. Then, as long as she interacted with the doll, she will have the option to use the doll as a decoy. Kaitlyn will lock the werewolf inside the freezer and save herself. Players will then earn the Final Girl Trophy/Achievement once the credits begin to roll.
For more guides on The Quarry, check out How to keep everyone alive in The Quarry on Pro Game Guides.