The ARMS are a series of Meld implant weaponry based off of corrupted Nth Metal. They’re all composed of different chemical compounds. The center mass is a mix of proteins and DNA, like you’d find in a virus. They are organic substances with certain organic characteristics a sentient super malleable metallic matrix.
Kiyama Harumi’s discovery of XCOM’s corruption prompted a faction or rebel soldiers and scientist to attempt to escape resulting in the creation of the computer Alice and the four ARMS core which the rebels were able to smuggle away in order to form a group known as EXALT who sought to defeat the evil actions of their former employers. A number of attempts had been done to recreate those cores using Alice which failed. However, the enemy of XCOM known as EXALT had discovered that by implanting the core into a female host in fuses the host embryo. Thus, they were responsible for engineering the birth of the ARMS children with this part of the project being headed by Nabiki Hibiki. The ARMS within the four children were all given names from Lewis Carroll's "Alice" books such as Knight, White Rabbit, Gryphon and Jabberwock. It was claimed that XCOM abandoned further research in the prototype ARMS with their scientists instead creating a new series that were devoid of their own intelligence which were implanted in hosts such as Keith Red.
Rumors were present within XCOM on the origin of the ARMS with some believing them to had been developed as a defense weapon. However, there were classified files in the organization that contained more information on the true nature of these machines. XCOM sought to recover the devices even if meant the death of their hosts. Godai Hibiki was one such subject who had it in his left arm and learnt under his grandparents to control it. He decided to seek out the agents of the XCOM and eliminate where he was guided by the sympathetic vibrations of his ARMS. This led to him mistakenly believing Ryo Saotome served XCOM due to the vibrations he felt from his ARMS. By this point, XCOM dispatched their operative Claw to retrieve the weapons but he was defeated. This chain of events led to Saotome and Hibiki working together to find the remaining two ARMS children in order to fight back against XCOM. Afterwards, the pair met a fellow ARMS user in “Fill” whereupon they came under attack from the Chapel Twins who used MEC-4 Alastor’s against them. The true nature of the ARMS was revealed to the three teenagers and the Jabberwock manifested within Ryo after he witnessed the perceived death of Belldandy.
Afterwards, the three children encountered the fourth female ARMS host, Spring Saotome who was working for EXALT.
They were considered products of metatechnology and were technological artifacts of incredible power that were not only strong but provided their hosts the ability to transform the respective limb containing the ARMS. These implants were described as a self-guided, synthetic stem cell. They were sophisticated devices that reshape themselves to mimic different cells, depending on where in the body they are. Blood cells, muscle cells, skin cells. They can even work together to imitate nerve cells an artificial neuron if you will. The first ARMS cores were in actuality bio-implants that when implemented in a female body fuses with their embryo. Once fertilized they decipher the genetic coding of their new host and were thus able to use that data to camouflage as well as sustain themselves. However, XCOM was unable to recreate those implants the one and only test subject ended up completely deformed as well as unable to function. However, using Alice they were able to create DNA specific Meld’s based off of the noted on the corrupted Nth metal. Once they are implanted in a host they start multiplying and fixing things supporting and enhancing their whole system. If anything goes wrong—muscle tears, injuries, whatever—the Melds will zoom in, multiply, and replace it until their own systems can catch up. An ARMS only manifested once the core chip was activated whereupon it abandoned its camouflage mode whereby in the span of seconds the Melds of the device multiplied in the body thus transforming the respective limb into a combat ready state. A more potent ability of these artifacts was the fact that they learnt from past experienced and continued to evolve as a result.
These devices seemingly nest within a random limb which were capable of transforming at a moments notice. However, one was present within the eyes of a subject which allowed her to see the smallest movements and activities of her enemies thus predicting their movements. Once activated, an ARMS appeared as a monstrous limb initially during its first stage where it possessed numerous traits such as enhanced strength, endurance and natural claws. An untrained host was unable to fully control an ARMS which reacted depending on the users emotional situation and thus capable of transofmring as well as acting on its own at any point. However, through proper training, a user was capable of not only harnessing an ARMS but controlling it in a more restrained manner. Some within XCOM referred to the users of such machines as possessing the "arm of a god". Though seen as implants, ultimately, an ARMS was an artificial intelligence that used binary numbers of 1s and 0s as part of its programming. Thus, it was vulnerable to computer viruses that attacked the functions of the AI. ARMS sought to repair all damage experienced by their host but were unable to fix psychological and emotional damage. Certain ARMS such as the White Knight and Jabberwock had the ability to disrupt the repair functions of the nanites and leave permanent damage to the hosts. Special Meld were also capable of being synthesized to make a poison called Venom that was designed to destroy an ARMS.
These nanomachines were noted to reach out to one another and created sympathetic vibrations amongst their hosts thus indicating the presence of another such ARMS. Those unaware that they had an ARMS implant experienced uncontrolled jerks in their respective limbs as if the user experienced a static shock. This allowed experienced users to determine the presence of another ARMS user which could either be a friend or a foe. Each ARMS was also able to exert the activation sequence in its counterparts with skilled users being able to accomplish such a feat. The Queen of Hearts ARMS was claimed to be capable of creating a synergy amongst the ARMS and even controlling them though she failed to be manipulate the uncontrollable Jabberwock.
Contact needed to be achieved with the ARMS in order to better control it and prevent it from going out of control. This required one to work in conjunction with it instead of fearing it. They were described as being highly versatile machines that responded to the desires of their host by adapting to circumstances. However, the ARMS ultimately had an intelligence of their own.
Despite their advanced nature, these devices were ultimately composed of Melds and were vulnerable to concentrated electrical shocks able to either kill or knock out the host bodies.