The Lockshield is not originally part of a video game universe that Alexander has interfered in, nor is it a weapon from the real world. It has, however, ties to the Kingdom Hearts universe and is based upon that universe's laws of working.
The origin of the Lockshield is never seen, but Yen Sid offers that the weapon was formed from the darkness of Alex's heart out of reaction to the Kingdom Hearts universe being created, as Alex would require a means of defending himself while traversing the worlds. As to the darkness the weapon is formed from, Yen Sid can only guess that it made up the majority of Alex's heart at the time.
It is capable of exactly the same tasks as the Keyblade, being its opposite number, which means it can perform tasks like locking and unlocking doors and chests, accessing World Keyholes, and defeating Heartless and Nobodies. Also like the Keyblade, the Lockshield can lock and unlock hearts. Masters of the Lockshield can even transform the Lockshield into a usable vehicle, also an ability of the Keyblade. It is summoned using the power of darkness, whereas Keyblades are summoned using the power of light.
Apart from this the Lockshield has a small range of other applications. It can be used to seal and create Dark Corridors, which Alex uses to return home to the real world. After The Interference, Alex can use the Lockshield to access other game universes, which are created and become amalgamated into the World.
Apart from its mystical capabilities, the Lockshield is used to defend from attack, and can also be swung or thrown. Though primarily considered a magical force, being able to be utilised specifically to kill magical enemies, it can still be used as a deadly physical force, and if so intended can be used to kill.
A weakness of the Lockshield is that, due to its heavy connection to the wielder's heart, the keyhole in the centre is a small exposed weak point, and an easy way to attack the wielder directly, though it takes either magic or skill to attack it.
Like the Keyblade, the appearance and abilities of the Lockshield can be altered through applications of various designs, although these designs do not take a physical form. Unlike the Keyblade, however, each design of the Lockshield is incredibly powerful, offering the user access to the type of magic found in the world of its origin, whereas each of the Keyblade's Keychains only offer different Strength and Magic stats and a special ability. There have so far been six designs.
The Heartless Seal is a design first seen in The Interference and is based on the Kingdom Hearts universe. It is described initially as being a small, round shield, with a dark green background, a sharp silver bladed edge, and a Heartless emblem in the centre. It is capable of granting the user a full range of magics and special abilities from Kingdom Hearts, making it and the Keyblade almost functionally identical.
The Kingdom Copy is a design first seen in The Interference and is also based on the Kingdom Hearts universe. It is briefly described as being a dark gold in colour, with a silver Hidden Mickey in the centre. As its name indicates, it is identical to the Heartless Seal and the Keyblade in terms of abilities, and very similar to the Heartless Seal in appearance.
This design is first seen in The Interference II: Curse of the Crimson Elixir and is based on the Fullmetal Alchemist universe. It is described as a dulled silver, and is emblazoned with a red Flamel symbol. It can grant the user the ability to use Amestris's form of alchemy in other worlds, but it is necessary to have crossed through the Gate in order to use it to its fullest potential; otherwise, it cannot work.
This design is only seen in The Interference III: Wings of Time and is based on the Chrono Trigger universe. It is a black shield with zigzagging gold lightning bolts bursting from the centre and over the edge of the shield, providing a serrated blade edge. It could only utilise Lightning-type magic, as characters in that universe only had a specific type of magic to each person.
The Time Fracture first appears in The Interference III: Wings of Time and is based on the Chrono Trigger universe. It has the appearance of being made of old stonework and features golden 'clock hands' and a bladed edge. Created from the Lightning Guard, it is capable of using Lightning-type magic as well, but its main abilities are to do with time manipulation.
Seal of MarEdit
This design first appears in The Interference IV: Dark Renegade and is based on the Jak and Daxter universe. As the name suggests, it is designed after the seal of the legendary hero Mar. Unlike the other Lockshields, it doesn't appear to be able to channel any specific abilities, as its wielder Alex's abilities from that world stem from edits made to his own body instead.
As the weapon was originally created for Alex, its spread is not nearly as wide as the Keyblade, being limited to a few key individuals.
As the weapon of Alex's heart, it serves as his primary form of defence. He (or rather, his heart) is the originator of all the designs above, and can personally wield five of them. His basic design is the Heartless Seal. As mentioned above, the Lockshield is spawned from his heart, so he has the most designs available to him, and for a long while was the only wielder of the weapon. Many of the other Lockshield designs have origins very specific to Alexander as well:
The Crimson Catalyst was created when Scar attacked him and aimed for the centre of the Lockshield, causing him to be sent to the Gate and pay the toll of both Joumae and much of his remaining light, losing him many of his Kingdom Hearts-related abilities. The new design served as a placeholder and reminder of what he had been through.
The Lightning Guard was created when Spekkio granted Alexander the power of Lightning magic. During the ceremony, Alex summoned the Lockshield and allowed some of the Lightning magic to enter the shield, combining itself with his heart.
The Time Fracture was created when Alex himself drops one of the surviving parts of the crashed Epoch time machine, a small computer chip, into the centre of the Lightning Guard. The Lightning Guard combines itself with the chip to create the new Lockshield, losing its status as a single entity in the process.
Originally a part of Alex's heart, and calling himself as the will of the Lockshield itself, Joumae has access to nearly all of Alex's designs and is an extremely capable wielder, considering himself a Lockshield Master (equivalent to the Keyblade Master role). Unlike Alex, he can unlock hearts from the beginning, and is capable of transforming his Lockshield into a Lockshield Glider. His basic shield is the Kingdom Copy, which replaces Alex's Heartless Seal in his roster, and was originally seen in Alex's heart as his weapon there, though it wasn't capable of changing designs.
Even is revealed to briefly wield the Kingdom Copy while Alex is unconscious. He is not seen fighting with the shield, but instead aiming to kill Alex with it instead. It is implied that the Kingdom Copy is the form in which Even extracted Joumae from Alex.