Calypso is a goddess. She is the daughter of Atlas, the Titan who carries the world on his shoulders. Her powers over Odysseus stem from her divinity, and we see just how much power she has as a deity when she twice offers to make Odysseus immortal. She is in love with the hero, and only allows him to leave when commanded by Zeus. Perhaps the most important aspect of this part of the story is not her power over the mortal Odysseus, but rather his willpower in turning down the opportunity to live forever with the beautiful Calypso:
Goddess and queen, do not be angry with me. I myself know that all you say is true and that circumspect Penelope can never match the impression you make for beauty and stature. She is mortal after all, and you are immortal and ageless. But even so, what I want and all my days I pine for is to go back to my house and see my day of homecoming. And if some god batters me far out on the wine-blue water, I will endure it, keeping a stubborn spirit inside me, for already I have suffered much and done much hard work on the waves and in the fighting. So let this adventure follow.
It is another example of human agency working both in tandem with, and in some ways against, divine will. The seductive power of Calypso is no match for his desire to return home.