Elizabeth complements Henry's Romanticism. She sees life in much of the same optimistic manner that he does. Both characters put others before themselves and look for the good in life (Elizabeth constantly encourages Victor in his darkest hours, as does Henry who even puts aside his own studies to help Victor get better).
In contrast, Elizabeth and Victor are foil characters. Victor is selfish and full of hubris. He seeks to bring glory to himself and fails to take responsibility for his actions and choices. Elizabeth is the complete opposite. She stays in the background taking care of her family members, selflessly giving of her time and love (much like Victor's mother Caroline). Moreover, she seems to take responsibility for events which are not her fault. For example, she blames herself for William's death because she gave him the portrait of Caroline which was taken from him by his murderer.