Elizabeth Lavenza can be defined as a foil to Victor Frankenstein (Mary Shelley's Frankenstein). A foil is a character whose characteristics contrast another character so that the characteristics can be highlighted.
Initially, Elizabeth foils Victor regarding her adoption into the Frankenstein family. Victor is the natural born child of the Frankensteins. Elizabeth has been saved from poverty and brought into the home. Here, Elizabeth is both female and not a natural child of the family. These characteristics are important when examining some of the themes of the novel: nature verses science, appearance verses reality, and nature verses nurture.
Later, Elizabeth's love of all natural contrasts Victor's love of all scientific. Elizabeth is satisfied with the world around her not changing (the mountains, lake, and scenery of her home)--it is important because it contrasts Victor's desire to change the world.
Lastly, Elizabeth's concern for Victor (when he is obsessed with reanimating life) exists because of her love for him. On the other hand, Victor's concern for himself lapses; he fails to concern himself with his own life (when trying to create another). In essence, his obsession with science is again contrasted with Elizabeth's love for nature.