The Guyver Units, also known as G-Units, are mysterious, symbiotic bio-organic mechanisms that bond with a host, enhancing the host's own capabilities. The Guyver Unit was designed as an all-purpose environment suit for its Creators (believed to be a group of aliens of different species). Although referred to as 'armor', the G-Unit changes many aspects of the hosts internal systems, replacing parts and even removing unnecessary organs. Besides increasing the powers of a human being when he/she wears it (more than when the Creators themselves wore the armor), it also frees them from the will and telepathic commands of the Creators themselves. This led to the naming of "Guyver", which in the Creators langauge means "Out of Control".
While extremely hardy, there are a number of things that can damage or destroy a Guyver. The various Enzyme-type Zoanoids were specifically developed to exude a type of acid that dissolves the Bio-Booster Armor. A battle-trained, experienced Zoalord has enough pure power to destroy a Guyver, as do some of the Hyper Zoanoids. Finally, the Creators developed a tool called the 'Unit-Remover'. It actually deletes host data from the Control Metal, forcing the Guyver to return to its inactive state. The condition of the human host after being subjected to the Remover's effects is unclear. Unlike the Zoanoids, the Guyver is humanoid in form - mostly because all known hosts are human. The Guyver would have been used by the command crew element of Creator spaceships and in later chapters, Shō uses his Control Metal to speak telepathically with one of the craft.
A Guyver Unit itself is semi-sentient and can act on its own to a limited extent if its host is unable to direct it. In this mode, it will usually take no action except to defend itself from what it perceives to be a threat. Guyver I was in this mode when he first assumed his bioboosted form, when he was regenerated by the Control Metal in Kronos headquarters and when he killed the Zoanoid his father had been turned into. It is presumably also this semi-sentience that prevents the host from disengaging the armor if they have sustained injuries that would prove fatal without the Guyver's protection and abilityto rapidly heal from damage. Guyver I has demonstrated the ability to regrow approximately 50% of the host's brain and skull in a matter of minutes.
The first Guyver was an experiment by the Creators eons ago. They were curious as to how their newly developed weapons would react to their standard armament (Guyver Unit). However, the Guyver (dubbed Guyver 0) attacked the Creators, destroying one of their ships. The human would not remove the armor when ordered to. The Creators programmed the humans to be telepathically controlled by them and so, as this human would not listen, the bio-booster human was named 'Guyver' (a word meaning "out of control" in the Creators' language, though other versions give it as "beyond the norm" and Bioboosted Armor Guyver gives the meaning as "not to specification"). They gave the 'Unit Remover' to Archanfel who then proceeded to remove the Unit from the human and incinerate him.
The first Zoanoids were created by the Uranus, approximately 110,000 years ago, as the final outcome of their bio-weapons development program. It is unknown if they were always a design goal, or a modification of the human form that was decided upon later. The latter seems likely, since not all 'original' humans possessed the ability to take on a Zoanoid battleform naturally; they had to be processed in order to gain this ability. Only a minority of mankind had been 'zoaformed'. The Uranus were said to be highly pleased with their Zoanoid creations, but after the Guyver-Zero incident, nevertheless abandoned them and attempted to destroy them.But mankind survived, thanks to Archanfel; the Zoanoids, now without a leader, intermingled with the rest of mankind. While their blood thinned out, the Zoanoid genes would resurface from time to time, leading to legends of monsters and demons. Finally, about 400 years before the present, Archanfel re-awoke and created Chronos, and eventually Zoanoid production began again (although the mass processing of humans into Zoanoids seems to be recent development).
After X-day, becoming a Zoanoid was actively promoted by the new government, offering tax incentives as well as the other benefits that comes with being zoaformed: Zoanoids are naturally more strong and able than the average person, don't get sick and have the ability to take on a super-powerful battleform which, in an emergency such as a roadside accident, can be extremely useful. Few people realize that Zoanoids have one more trait: they are completely subject to a Zoalord. They obey the telepathic commands of the God Generals without question, even across great distances. The horrific consequences of this became apparent when thousands of ordinary, but processed, people got up and walked away, to merge with, and die with, the Cabraal Khan.
It appears that a Zoanoid can pass on their ability to transform to any offspring they produce; if only pure Zoanoids were to interbreed, processing might no longer be necessary. Oddly enough, no female Zoanoids - or rather, women who can take on a Zoanoid battle form - have ever been seen.
Sho Fukamachi(深町 晶 Fukamachi Shō) is the main protagonist of the series, Shō is seventeen years old and a second-year student at Narisawa high school. He has black hair and a slim build. He fights Kronos more in order to protect friends and family than from any need to see the enemy brought to justice. Had he not become host for a Guyver Unit, he would have happily continued in ignorance of Kronos. He cares deeply for Mizuki Segawa with the intensity of first love and Tetsuro Segawa, Mizuki's older brother, is his closest friend. As he is the host for the first Unit activated, he is known as "Guyver I".
Oswald A. Lisker(オズワルド·Α·リスカー Ozuwarudo Ēi Risukā) or simply known as Lisker, is a Kronos agent, or as he describes himself to Shō, "an inspector from Kronos HQ". Lisker was in Japan to supervise the transfer of the inactive Guyver Units to Kronos headquarters and was considerably annoyed to discover Makishima had managed to lose them. When one damaged unit was recovered, he insisted on taking a look at it and involuntarily became its host. As the second known activated Unit, he was called "Guyver II".
Agito Makishima(巻島 顎人 Makishima Agito) is of medium build and muscular; he is a third-year student at the same high school the others attend, but looks older. Portrayed as an anti-hero, he almost never allows sentiment to stand in his way. He appears shrewd and calculating. At one point a candidate-member of the Kronos Elite, he is now viewed by them as a traitor. Even though he may have activated his Unit before Oswald A. Lisker, the second Guyver, he did not openly appear in his Guyver armor until after Lisker was eliminated and thus he was dubbed "Guyver III". Agito shows a frightening knowledge of what the Guyver can do throughout the series.