The Lock is a substandard version of The Shield, as it has the ability to 'lock' anything so it can't be unlock through conventional means. It also blocks all possible exits, such as if it 'locked' a room, all the windows and other doors would be locked/impervious to attacks as well, disrupts any magical senses, like The Maze, and can protect its true form with a psycho kinetic storm. The Lock has a weakness that it has to inhabit an actual lock to 'lock' an area. It's true body is a green padlock with gold highlights and a wing on each side of it's keyhole. It is under The Earthy.
The Lock first appears in Episode 38. Sakura and her class went to a strawberry plantation to pick strawberries for a class fieldtrip. After they picked strawberries, Naoko tells Sakura and Meiling that the room they were going to bake strawberry cake in is locked and would not open. Sakura, Syaoran, Tomoyo and Meiling check to see if the room really was locked. Strangely, they find the door wide open and enter, but then the door closes behind them and they are locked in. Ms. Mizuki sensed that the four were trapped by the card and helps them by telling Sakura to 'unlock' the door, gesturing to Sakura's Clow Key. Sakura used the Clow Key on the lock and the main body of The Lock popped out of the keyhole. Lock tried to make a dash back into the keyhole, but Syaoran sealed it off with one of his wards, exploiting its weakness. With its true form captured, Sakura sealed The Lock back into its card form.
The Lock is transformed in Episode 53. It was one of the many cards in Sakura's panicked rush of Sakura Card transformation.