Persephone is the daughter of Regina. Her father is unknown.  But  unknown to anyone in Storybrooke, Rumple wants the 14 year old daughter of Regina to become the next dark one. So upon his request, she kills him. But one day Persephone accidentally leaves her dagger, since it fell out of her purse, only to have Belle pick it up and have Persephone become her servant. Forever. And Regina doesn't object to this because all her memories of Persephone have been erased