Before the apocalypse started, Rainier was spending a life sentence in a maximum security prison. Thankfully- or sadly, depending on how you look at it- the very cause of this apocalypse was located in a government facility that happened to be very nearby.

Theo worked at said facility. He was involved in some extremely shady, inhumane experiments that made him something more than human. He and his colleagues lost control of the situation, unleashing a zombie apocalypse upon the world.

There were already several dozen zombies that had once been trapped in the facility.  And this was in the middle of the day, so there were plenty of victims to add to their numbers. By the time the zombies reached the prison, there were well over 500 of them, flowing over and devouring anything in their way. Rainier and some of the other inmates managed to escape in the chaos of the horde's attack. In the process, though... she was infected. She broke off from the group and stumbled through the evacuated city, growing more delirious by the second.

In some strange twist of fate, she met Theo. He purged the virus from her body, giving her the same immunity to it that he had. They decided to team up and watch each other's backs; over time, they grew closer and closer, eventually becoming the inseperable, unstoppable duo that we know and love. :)