One of the Princesses of Heart, her world was destroyed by Maleficent. She was taken and held deep within Hollow Bastion until all seven Princesses were collected. It is unknown when she was captured but from scenes it can be assumed it was before Snow White and Alice placing her as either 2nd captured or 3rd either before or after Belle. She and the other princesses upgraded Sora's Fire spell to Firaga. She first appeared in Hollow Bastion in Kingdom Hearts.
When Kingdom Hearts was sealed she was returned to her own world.
Cinderella is absent in Kingdom Hearts II. (though her name was part of the password that allowed access to the DTD dataspace in Tron's world) She will, however, return in the upcoming PSP game Kingdom Hearts: Birth by Sleep.