The Jabberwock is the nanomachine weapon system bonded to the series' main protagonist, Ryo Takatsuki, manifesting primarily in his right arm. It is one of four original ARMS units created by the clandestine organization Egrigori. The Jabberwock originated from the fragmented psyche of a young prodigy named Alice, embodying her intense hatred and destructive impulses, while a counterpart entity represents her will to protect. This artificial intelligence is characterized by extreme violence, instability, and a single-minded drive to destroy.

The personality of the Jabberwock is that of a demonic entity with no purpose other than annihilation. Its rage is such that it can feed on and reinforce the anger of its host, Ryo, creating a cycle that allows for easier synchronization but also risks losing control. When Ryo is in grave danger or suffers extreme emotional distress, the Jabberwock can seize control, triggering a full transformation into a hulking, demonic humanoid form of pure destructive force. Its motivation is fundamentally tied to its origin as the embodiment of Alice's hatred for humanity, making it a relentless engine of carnage when unleashed.

Within the story, the Jabberwock serves as both Ryo's greatest weapon and his most significant threat. Initially, Ryo views the Jabberwock as a dangerous power to be suppressed at all costs. However, as the narrative progresses, their relationship evolves from one of parasitic dominance to a reluctant symbiosis, and finally to mutual cooperation. A pivotal moment occurs when Ryo accepts the Jabberwock as a part of himself, convincing the entity to follow his will rather than seek to dominate him. This development allows the Jabberwock to overcome anti-ARMS viruses and unlock new abilities, such as superheating its body. The Jabberwock's rampages often necessitate the intervention of the other original ARMS users, Hayato Shingu (White Knight) and Takeshi Tomoe (White Rabbit), who are prepared to destroy it if it cannot be controlled. Hayato’s Knight is given the specific mission to act as Ryo’s executioner should the Jabberwock fully consume him, creating a poignant conflict between friends. Ryo’s deep connection to his childhood friend, Katsumi Akagi, is a crucial emotional anchor; her apparent death serves as the catalyst for the Jabberwock's most devastating awakening, while his desire to save her provides the strength to master its power.

The Jabberwock’s most defining ability is its capacity for rapid, reactive evolution, granting it a new power to counter virtually any threat it faces in combat. Its initial capabilities included elongating its limb, forming a shield, and accelerated healing. One of its early significant evolutions is the compressed air cannon, which consumes surrounding materials to create and fire high-velocity projectiles, giving Ryo long-range firepower. A signature and exceptionally dangerous power is the anti-ARMS nanovirus present in its claws, which can permanently disable the regenerative nanomachines of other ARMS units on contact; even broken-off claws retain this lethal property. Other notable evolved abilities include electromagnetic distortion to deflect energy attacks, immunity to space-time manipulation, hypersonic vibrations that shatter matter at an atomic level, and limited flight via thrusters. Its most catastrophic power is the ability to generate anti-matter internally. Unleashing even a minuscule amount can devastate an entire city, and it is stated that the Jabberwock has the potential to create enough anti-matter to destroy the entire planet. Ultimately, through Ryo’s willpower, the Jabberwock’s path leads from a force of pure hatred to a controlled, symbiotic weapon, culminating in a final confrontation where it helps resolve the schism within Alice’s soul.