Bella swan is not a mermaid. She is a human being, And she is the main character of the New York Times #1 Bestselling Novel, Twilight, written by Stephenie Meyer. Isabella Marie Swan is portrayed by Kristen Stewart. She is in all four movies of The Twilight Saga. And she is human out of all three movies. In the fourth and last movie, The Twilight Saga: Breaking Dawn Part 1 And 2, she becomes a Vampire.