Before the game
Prior to meeting Vashyron and Zephyr, Leanne was a guinea pig for a research experiment run by Sullivan, one of the games main antagonists. Sullivan's goal was to manipulate the life span of human beings. With that, he took 20 people (Leanne included) and assigned quartzes (stones that can be programmed to change life span of people) to them. Leanne was named Subject 20, probably because she was originally programmed to die at the age of 20. However, Juris, a scientist working for Sullivan, felt sorry for Leanne and her eventual fate. Once the experiment was done and the results were collected, Juris planned to set Leanne free and let her live out her last year of her life and see the outside world (Leanne was 19 at the time). She could've been stopped by Rowen, a cardinal (an elite member of the Basel community) who disproved of Juris' actions and almost had Leanne stopped and taken away. However, following the words of his late friend Freida (who wanted to see people live life as they please), Rowen lets Leanne go. "Her fate is the same either way", he states.
Hearing that she will die in one year, Leanne decided to end her life on her own terms. On the day she was programmed to die (Christmas Day), she attempted to jump off a platform from a great height, but during her descent she was rescued by a boy named Zephyr. For some odd reason, Leanne didn't die as she was supposed to, and both she and Zephyr dove downwards towards the grounds of Basel (Why they survived the fall without parachutes of the like is not known). After they survived, Leanne followed Zephyr home and moved in with him and Vashyron. To this day, she believed that Zephyr saved her from her death.