Here's a breakdown:
* Twilight: They meet and begin a romantic relationship, despite Edward's vampire nature.
* New Moon: Their relationship is tested by Edward's departure and Bella's dangerous attempts to cope.
* Eclipse: They face external threats and reaffirm their commitment to each other.
* Breaking Dawn: They get married and Bella becomes pregnant with their half-human, half-vampire child, Renesmee.
Their relationship is complex, passionate, and full of challenges, which makes it so compelling to readers.