A magical lock used by Amu Hinamori to complete her Character Transformation.
The Humpty Lock was given by the Original King of the Guardians to Tadase Hotori, the present King's Chair. He was
The Key and Lock
told to give it to the Joker, a.k.a. Amu Hinamori. Amu is able to tap into the lock's powers to perform a complete Character Transformation (as opposed to Character Change, which any of the Guardians can do-- a partial change improving their abilities or character in certain ways, but less than a full transformation does). It is also thought that the key and lock have some sexual ideal but this has not been proven.
It appears that the Dumpty Key is intended to fit in the Humpty Lock, but it is unclear what conditions need to be met in order for this to happen-- in episode 38 Ikuto Tsukiyomi attempts to fit the key into the lock, but it apparently doesn't fit.