The key characters in the "Twilight" saga are Bella Swan and Edward Cullen. Bella is a human, and Edward is a vampire whom she meets when she enters a new high school.
Bella Swan is the heroine of the "Twilight" Saga and the viewpoint character of the books. Since she has moved from Arizona to Washington to live with her father at the beginning of the series, she has only recently met Edward Cullen as a student in her new town's high school and become aware of the existence of vampires. Bella falls in love with Edward and comes to crave the bite of a vampire, which would turn her into one.
Edward Cullen is the hero of the books and is extremely magnetic and attractive. He is unusual for a vampire hero, though, in being highly restrained and moral. He lives with several other vampires in a "family," but all of them only drink animal blood because they consider it wrong to feed on humans. Edward does not bite Bella; he thinks it would be immoral even if she wants it. He is attracted to Bella and easily enters into a romance with her, making him a romantic hero with a sense of morality.