Libby is the girlfriend Harry Kim left behind on Earth, with whom he found himself living again in an altered timeline.
Libby was Harry Kim's girlfriend in San Francisco before he joined Voyager's crew, and missing her was one of the hardest parts of his life in the Delta Quadrant.
In Non Sequitur, Kim woke up beside Libby in San Francisco after a shuttle accident, living a life in which he had never been posted to Voyager. Being with her gave him everything he thought he wanted, and she was hurt and confused by his strange behavior. Yet Kim came to believe this was not the life he was meant to live, and chose to recreate the accident in the hope of returning to his ship and crewmates.