“Tina, it's me. Your brother? Alex? Listen, mom would kill me if she knew I was contacting you, but she's being an impossible bitch right now. You have to come home. Do you understand me? This thing with China... it's not going away. We're going to try to get into one of those Vaults. If that doesn't work, well... I have a backup plan. There's this secure bunker... another sort of vault... inside the National Guard depot, and... Look, no matter what it takes, the whole damn Keller family's gonna ride this storm out, but you've got to get your ass home. Now!”— Alex, on his holotape
Alex Keller was part of the Keller family. He does not actually appear in the game but is mentioned in the Keller family transcript.
Alex was a National Guardsman, who stole the bunker codes, so his family was able to survive the nuclear holocaust. He was caught but before he was taken into custody, he sent each member of his family a single digit of the code. This way, they would all have to be there to get in.