Lizzie has a severe mental condition that leaves her sociopathic toward the majority of humanity and gives her a desire for a specific person to direct her full attention towards. Though she watched several people, the person she focused on was John Nichols. When he reciprocated her feelings, she became more obsessed with him, but at his insistence also more balanced, developing interests of her own.
Aside from this, Lizzie appears to be somewhat of a romantic, wanting things like a traditional wedding. She also has some normal opinions for a girl her age, for example having read that talking about marriage would "scare off boys" (she doesn't see the irony in saying this to someone she kidnapped).
Lizzie's primary weapon is an axe, but she also uses knives and appears to enjoy collecting them. Her favorite subject is chemistry, where she learned to make chloroform. Aside from John, she appears to have few interests that are not practical, completely uninterested in music or movies.
In approximately 1972, Lizzie's mother participated in a KoiTec experiment while pregnant with Lizzie, irrevocably altering her psychology and physiology. It's impossible to know how much of her later personality was nature and how much was nurture, but she had both a dysfunctional family (absentee father, addict mother) and signs that she was unusual from a young age.
When Lizzie was "very young" she helped her abusive mother overdose on unknown drugs, intentionally killing her. In elementary school, she killed a girl who was cruel to her, making it look like an accident. In middle school, Lizzie tried to keep to herself, but said this attracted attention from other girls. By high school, she appears to have learned to act more normal, finding a girl to pretend to be friends with.
In 1990, her last year of high school, Lizzie fixated on John. In the canon path, they started dating, but when John refused to sleep with Lizzie, she kidnapped him and held him in her basement over a week-long vacation. John escaped his bonds but remained as a show of trust in Lizzie, which deepened their relationship.
Unfortunately, they had attracted the attention of KoiTec, which captured John to use him against Lizzie. With assistance from John, Lizzie was able to kill all the KoiTec operatives and free him, then cover up the incident and escape.
The rest of their lives is unknown, though the epilogue an unknown number of years later suggests they lived happily together. Sierra has stated that if a sequel is created, it would take place in 1995 and focus on Lizzie and John after their marriage.
- Lizzie's name is probably a reference to Lizzie Borden. Supporting evidence includes both using an axe as a weapon and their mothers being named some form of "Abby." The origin of her last name is unknown.