Beth Johanssen played by Kate Mara was born on May 9th, 2006 in San Jose, California. Her father is the district sales manager of a napkin factory. She was selected to be an astronaut in 2032 with NASA Group 27. She was the mission sysop for Ares III and the 15th person to set foot on Mars.
In the book, had the Taiyang Shen probe failed and they didn't resupply, the rest of the crew would have killed themselves with pills and she would have had to consume their flesh for calories etc whilst The Hermes went back to Mars and then back to Earth as she was the youngest, smallest and the most able to make it back to Earth as she can run all the computer systems. She told all this to her father before the probe was launched.
She developed an aptitude for computers at an early age, graduating from high school at age 16 and went on to win NASA's hackathon at age 17, before moving on to MIT to do dual undergrad degrees in Maths and Computer Science.
Before graduating Stanford University she had started her own private software company and went to Silicon Valley in hopes of becoming a software engineer and CEO. However, after talking to an executive at Space X, Brett Parker, and then working on one of the systems that would become integral for The Hermes (on his suggestions as he had been impressed by her work), she switched career paths and was inspired to work for NASA.
She was able to leverage a spot on Ares III because of her prior knowledge of The Hermes systems.