She is as harsh and untamable as the sea itself, she was in love with Davy Jones, and she gave him the duty of ferrying souls to the afterlife, giving him The Flying Dutchman to travel across the dimension that became known as Davy Jones Locker. In reward, he would have been reunited with her for a single day.
Jones kept his word and did his duty for ten years, but Calypso never waited for him, and in anger, he carved his heart out and locked it in a chest. Because Jones abandoned his duty, he became a sea tentacled monster.
In revenge, Jones agreed with the Brethren Court to imprison Calypso in a human form.
Calypso then became known as the famous voodoo priestess Tia Dalma.
Calypso was then freed in at the end of the 3rd chapter At World's End, by the Brethren Court agreeing on freeing her to win against EITC. When Calypso was free she summoned a storm followed by an enormous whirlpool which would later carry Jones inside it and then Calypso would have calmed it down.