In traditional Greek Mythology, Persephone was abducted to the Underworld by Hades and was forced to stay there as his queen. However, Zeus and Demeter made Hades agree to only keep her for 3 months out of every year and let her go for the other 9. This said to be the reason why there is a Winter season.
Story of Joining Sora's TeamEdit
While she was being captured by Heartless like some of the other gods, she was freed by Sora, Kairi, Lock, Shock, and Barrel when they found her in the underworld. But right before they could escape the underworld, Pete and Hades stopped them and had a showdown with Sora, Kairi, and Barrel, as Lock and Shock guarded her. When the fight was over, she was greatful that she wanted to become an honorary member, and asked Sora if she could join the team.
On Sora's TeamEdit
As she's the goddess of innocence, she became the team's judge to see the heart in others to tell of those are gulity or innocent.