Who steals the key to set Cinderella free?

The key to set Cinderella free is not stolen in the traditional Cinderella story. It's actually Cinderella herself who finds the lost glass slipper, which acts as a symbol of her identity and helps the Prince find her.

The stepsisters, in their jealousy, try to fit their feet into the slipper, but it doesn't fit. This allows the Prince to recognize Cinderella as the true owner of the slipper and, ultimately, the woman he loves.