The Guyver is a symbiotic alien life-form engineered to be the ultimate biological weapon. Referred to as a "bio-boosted armour", it was designed by a group of beings known simply as the "Creators", alien beings that once roamed the stars and were responsible for engineering life on Earth. These "Creators" were responsible for creating the human race as the basic model for their ideal living weapon and were curious as to how a human being would respond to being bonded to one of their own standard symbiotic armaments. The Creators were terrified by the result: a human bonded with one of their standard armaments gained unparalleled strength and combative ability, but they were completely free of the Creators' telepathic influence. Fearing the capabilities of the bio-boosted human, the aliens dubbed this abnormality as out of control, or in their own language, "Guyver".
Before bonding to a life-form, the Guyver appears as a rounded bio-mechanical object shaped somewhat like a land-mine. The device appears as a series of organic tendrils or cables encased within a set of metallic braces, with a gleaming metallic core in the centre. This core is the Guyver unit's control mechanism and will react to the touch and brainwaves of a potential host. The G-unit will only bond with a sentient life-form, something capable of learning its functions and using them effectively.
When a G-unit is activated, the fleshy coils inside the metallic braces will writhe and stir. Afterwards, the braces will release and fall off before disintegrating. The fleshy tendrils of the unit will then envelop the new host, joining with its cell structure and re-configuring its own mass around the host subject. Once the bonding is complete, the subject will be encased in organic armour plates on top of an epidermal layer consisting of the same tendrils as the unit's internals, tightly bound around the host.
The Guyver armour may feel soft and leathery to touch, but it is actually highly shock absorbent and cannot be damaged by conventional weaponry. The Guyver increases its host's natural strength by several magnitudes as well as the subject's speed, agility and stamina. The host's reflexes and senses are also heightened to an astonishing degree with the addition of two sensor globes on the host's "helmet".
The bio-boosted armour is supported by a control medal (the mechanism in the centre of an inactive G-unit) which regulates the Guyver's power output and prevents it from overloading. The medal appears on the head of the user and is connected directly to the brain when the armour is in use. Damage to the control medal can cause the armour to malfunction, perhaps even deactivate in the middle of battle. If the medal is critically damaged or outright severed from the user's head, the Guyver will overload and devour its host alive, causing the user to rapidly decompose as the body's cells consume one another.
The Guyver's host can freely activate and deactivate the armour at will. When the armour is not in use, the host's body will appear as it normally did before being joined, with the addition of two small growths on the host's back. These growths are some form of extrasensory organ that allow the host to summon the Guyver when needed; the armour itself seems to exist in some kind of extra-dimensional space when not in use, like a pocket dimension. This organism can also be used to communicate telepathically with other bio-booster users.
The Guyver armour releases and vanishes when the user wills it so. To re-equip the armour, the user normally states an activation keyword, a specific word like "Guyver" or "Bio-boost".
As well as enhanced physical strength, a Guyver is also equipped with a number of extremely powerful offensive weapons. These weapons and their effects are listed below.
- Head Beam - A highly focused infra-red laser that is fired from a tiny gem resting just above the control medal on the user's forehead. This beam can swiftly burn through steel and concrete.
- High-Frequency Swords - Each of the Guyver's arms is equipped with at least one blade protruding from the forearm (Guyver-III has two blades on each arm). These blades vibrate at tremendously high-frequencies and disrupt the molecular bonds of the objects they touch, then cut through them easily. These blades are capable of cutting through just about anything.
- Gravity Control Orb - The metallic orb attached to the Guyver's waist controls gravitational forces, allowing the Guyver to levitate. It can also allow a Guyver to generate micro black-holes to use as an offensive weapon. This technique is known as the Pressure Cannon.
- Sonic Buster - The Guyver's mouthpiece can emit devastating ultrasonic waves that can completely shatter targets like glass.
- Mega-Smasher - The Guyver's most powerful weapon. The mega-smasher is an immense particle beam cannon that unleashes a blast capable of vaporising everything in its path. At full output, the beam is roughly three feet wide and has about a mile's range. The mega-smasher is fired from two gelatinous nodes concealed beneath the Guyver's chestplate, which must be pulled open by hand for the beam to fire. The beam can be fired from just one side of the chest should the user choose to do so.
The Guyver is capable of healing injuries extremely quickly, even those that would be fatal to ordinary humans. If given ample time for recovery, the Guyver can completely restore severed limbs and damaged organs, even the brain. As proven by Sho Fukamachi, the first Guyver host, it is even entirely possible that a Guyver host becomes biologically immortal by joining with the G-unit. Sho himself had his control medal removed from his head and his body completely dissolved as a result, but because the control medal was undamaged and kept intact, it was able to reconstruct Sho's entire body from just a few flecks of blood left on the medal's fibers. It was even able to completely restore Sho's memory from up to the moment he was killed.
If a Guyver host is critically wounded and rendered unconscious, say from having his brain damaged, the armour will act on its own accord to protect the life of its host. In this state, however, the Guyver cannot distinguish between friend and foe and will mindlessly attack anything it considers to be a potential threat. The armour will act in this manner until its injuries fully regenerate and the user regains consciousness.
Only three Guyver units were known to exist on Earth.
- Sho Fukamachi - A Japanese high school student who happened upon a unit by accident during the D-59 Incident. After he picked up the device, it activated and bonded with him. Since then, Sho has used the power of the Guyver to protect himself and his friends from the machinations of the secret organisation called Cronos.
- Oswald A. Lisker - An agent of Cronos from the United States. He came to Japan to oversee the recovery of the stolen Guyver units, but only one was recovered and had been damaged during retrieval. Lisker inadvertently bonded with the damaged unit and used it to try and apprehend Sho Fukamachi, but the armour malfunctioned and it consumed him.
- Agito Makishima - A high school student and former Cronos management candidate. He recovered the third unit and activated it as his means of destroying Cronos from within, but his rebellious plans were exposed and he has since opposed them openly alongside Sho and his friends.
- Valkyria - A female Cronos agent who stole an artificially manufactured control medal and bonded with it, becoming Guyver-II F. She is currently aligned with the rogue Zoalords against both Cronos and Sho Fukamachi's resistance.