Note: I think of Hades as pain rather than darkness.
The light girl, the one which laughs at the darkness in hopes that it goes away, is young Kore, denying the frightening darkness within her and pretending to be perfect.
Her mother leaves, and she is abandoned by her friends, just for a moment. In that impossibly short period of time, Kore is left completely vulnerable. Hades takes the chance to lure the girl down with the narcissus flower, a promise of beauty and perfection.
Kore is swallowed up by the ground and dragged kicking and screaming to the Underworld. A part of her knows she belongs there, but it is hard to accept. The Underworld is no place for perfect girls, instead it houses darkness. Hades rapes her mind, and for a while everything Kore does centers around Hades.
She refuses food and slashes desperately at her skin, trying to run from Hades. It doesn't work, though. Hades is always there, no matter how clouded the world is with hunger and physical pain.
Then Kore tires of running, and looks Hades in the eye. She realizes that controlling Hades is possible, because she is more powerful than he is. It was him that had fallen in love with her in the first place, was it not?
Kore takes the pomegranate seeds offered to her at the very beginning, and eats them. All six. She knows the consequences of her actions, knows she can never fully go back. She doesn't want to go back to spring like nothing has ever happened, doesn't want to be numb and perfect, the knowledge of pain but a dark shadow long left behind. The pomegranate seeds are her salvation from that.
Kore is at peace with Hades, and has accepted him and loves him because he is a part of her. That doesn't mean she wants to be with him forever. She misses her mother, the butterflies and flowers, because even though there is darkness in her, there is still light, an extraordinary amount of it.
So she goes back, no longer the innocent Kore she left as, but beautiful Persephone, who knows and has tamed her demons through strength through the pain she still does know and has grown to accept as part of her.
And as long as Persephone lives, the pain will always be there, but she is strong, unbroken, and it has no hold on her. She is the queen of the underworld and all the darkness there, but also the goddess of spring and the hope it brings.
