Katrina has the job of greeting and helping strangers understand the community. She was raised in Vault 15, in which she had a very boring life. Over time the vault that Katrina lived in became overcrowded, forcing the residents to leave. Everyone gathered their supplies and ventured out into the wastes with no knowledge of the outside world. Apparently, a new life would begin for everyone. However, after her departure, Katrina was wounded in a raider attack and ended up in Shady Sands. She has stayed there ever since.
The intention of the developers of Fallout was likely to include her as a tutorial character considering her dialogue is focused around explaining a few fundamental game-play systems, including bartering, healing and the First Aid and Doctor skills.