Elizabeth is adopted by Caroline Frankenstein when she is five from a family of poor Italian people. She is beautiful and sweet. She is supposed to marry Victor. When Justine is accused of murder, she believes in her innocence and speaks on her behalf. She writes Victor letters, and worries that he has fallen in love with another woman. But no. Victor marries her, and she is murdered by the monster on her wedding night.
Elizabeth is beautiful, so, based on Victor Frankenstein’s logic, she must be a good person. Much as the monster is defined by his ugliness, Elizabeth is defined by her attractiveness. This is most likely why Victor is all happy to marry her. Much like the murder of Henry, the monster kills Elizabeth to drive Victor into isolation. Or possibly to get Victor all to himself.