Lock's visible form is a small green and yellow padlock with a front mounted drum. It has wings on the front, straddling the chamber of the lock.
Magic and Abilities
Lock Magic: The Lock is able to lock almost anything (doors, padlocks, etc.), also preventing their unlocking by conventional means. Lock can seal off entire rooms by magically shielding any and all exits such that they cannot be penetrated or destroyed.
Fortune Telling: All Clow Cards have the ability to help their user predict the future. Their method of fortune-telling is similar to that of tarot cards. The interpretation of the message the Clow Cards relay depends on the magical power of the user.
In Episode 38 of the anime, Lock inhabits a storage building (currently used as a kitchen and presentation room) at a strawberry plantation, where it traps Sakura and company until Sakura drives it out of the room's lock using her Clow Key. Lock immediately tries to reenter the keyhole of the lock, but Syaoran blocks the way with one of his wards. This reveals one of Lock's weaknesses, where it cannot simply merge with something in order to 'lock' it, but it instead must find a lock or catch from which to inhabit and activate its abilities.