Agito in canon
Once the foster son of a man named Genzo, Agito came to hate the company that the man worked for almost as much as he hated the man himself. Genzo, who had driven Agito's parents into bankruptcy and later suicide, was a greedy, power-hungry man, with seemingly no redeeming qualities at all. Certainly none that Agito could see, or even care to look for.
Agito grew to young-adulthood with the knowledge that too many mistakes, too much defiance, would make him useless to Genzo and he would most likely be killed.
The first time Agito appears in the story, he is seen presiding over a meeting of the Narisawa High School student council. Sho, the secretary of the student council, also makes an appearance.
In the 2005 TV series
As he has very little interest in mundane affairs, he cuts the meeting short and leaves quickly.
In the manga/12 part OAV
Another day presumably passes after that, because the next we see of him is at the end of school. He passes Mizuki Segawa and another, unnamed, girl in the halls, leading to the discovery that he is rather well-liked among the female population of that school. Stopping on his way out, he gave Mizuki a gentle reminder not to be late for the next meeting of the student council.
However, he was only interested in Mizuki insofar as he could use her to gain the cooperation of Sho, and thus to gain the use of his abilities by proxy. Agito has no real concern for her as a person, though he does put on a rather convincing act. He was once friends with her brother, though most likely just friends of convenience considering his personality, and he later inquires as to the other boy's health when he and Mizuki are leaving school together.
When he learns that Mizuki has been taken hostage by the Hyper Zoanoid Zerebubuth, through unknown means but it is likely he was informed by one of his unknowing dupes within Chronos itself, he took action to protect her. Killing the Hyper Zoanoid, he left after exchanging a few cryptic asides with Sho. Given his nature, it is likely that these remarks were calculated to make the younger Guyver more willing to see him as an ally.
The next time we see him is when Mizuki is leaving school; he speaks to Mizuki for a few moments, reminding her not to be late for the next meeting of the student council. Leaving her behind, Agito briefly muses on the convenience of Tetsuro Segawa's sister being so close at hand. Catching up with her later, knowing that Sho was close to her on an emotional level, Agito offered to walk her home in an effort to get under Sho's skin.
As observant as he was, it's more than likely he knew that Sho was watching.
He next appeared meeting with Mizuki, likely as not completing some work for the Student Council. Expressing concern for Tetsuro that may not have been entirely feigned, Agito was presumably able to discern just what Mizuki did and did not know about the Guyvers, their holders, and Chronos itself.
Agito is next seen as Guyver III; he comes to kill the Hyper Zoanoid holding Mizuki hostage - Zerebubuth - and therefore gain the trust of Sho and Tetsuro. This was, likely as not, so that when he either chose to reveal himself to them or was forced to by some unforeseen circumstances, they would both be more inclined to trust him.
Leaving behind nothing more than cryptic words and a dissolving Hyper Zoanoid corpse, Agito next appeared speaking with Richard Guyot about what was to be done with his adoptive father. The decision itself had already been made - Genzo was to be transformed into the prototype for a line of anti-Guyver Zoanoids that Chronos intended to unleash on Sho: the Enzyme series - but Guyot, for reasons of his own, wanted to know what Agito thought about matters.
Agito, who had never been particularly fond of Genzo on any level - the man having driven his real parents into bankruptcy and indirectly lead to them killing themselves - freely admitted that he had only been using Genzo for his own ends, and implicitly stated that he would be would be perfectly happy to discard him now that Genzo was no longer of use to him.
Guyot, clearly pleased with these sentiment - or rather the pronounced lack thereof that Agito had demonstrated - asked him if the other two teenagers that he'd often seen in the company of Guyver I were from the same school that Agito himself attended. Finding out that they were, Guyot gave Agito orders to bring them in.
As his aims coincided with those of Guyot and Chronos for the moment, Agito readily agreed.
The next time we see him, Agito - in his guise as Guyver III - is watching from concealment deeper in the forest as Guyver I fought to the death against the new type of Zoanoid that had been created specifically to kill him: Enzyme. Once Guyver I's Control Medal had been torn out, returning the Unit to it's natural, parasitic state, Agito waited and watched while the Control Medal was seized by Chronos Troops under the command of Guyot, and then found some means of following them.
Guyver III is next seen killing a group of Enzymes while they're being processed for combat, clearly knowing just what kind of danger he would be in if he allowed them to live; Guyver I's rampage gives him the perfect distraction to slip away, and Guyver III vanishes once more.
He reappears at the conclusion of a rather dramatic speech by Richard Guyot, seemingly chastising the man for attempting to play god with the aid of Chronos. He then introduces himself, telling Guyot that he could call him Zeus; stating that it was rather fitting, considering the topic that had been brought up previously. Before any other words can be exchanged between the two of them, a series of explosions rocks the building.
A nearby Chronos Trooper reports that the explosions were caused by a series of bombs that had been placed in various locations within the building. Guyot is quick to accuse Guyver III of having been responsible for planting said explosives, and Guyver III is just as quick to accept such an accusation. Seeming rather pleased with himself, Guyver III leaves.
His last words of the volume are directed at Guyver I; telling him to use his Gravity Controller to flee the building by air, and then rather pointedly ignoring a request for information on his true allegiance from the younger Guyver.
Agito reappeared in volume 2, reading a newspaper article about the destruction of Chronos' Japan branch building. Pleased not to have to tie up any further loose ends, Agito went to school for the day. There, unhappily for him, he was forced to confront the strange creature that had grown from the remains of Guyver I's left arm.
Not knowing precisely how to deal with the new situation he faced, he recalled his Guyver and went to confront the creature beside Guyver I. When the creature transformed back into a clone of Sho, sans the clothes it had presumably stolen, he was quick to advise Guyver I to continue his attack, and then to cripple both its legs. When the creature attacked Guyver I, however, he was fully prepared to obliterate both of them with his Mega-Smasher.
Watching in some surprise as the creature was re-absorbed by Guyver I's Control Medal, Guyver III retreated, reappearing in school as though nothing had happened.
Agito in Warrior Guyver
Agito in Son of Mine ('verse)
With no real changes to his past or personal history, Agito remains the same man that he has always been.
He was not initially willing to trust Masaki Murakami, and the realization that there was yet another thing that the man had hidden from them only made things more contentious between them. However, Murakami's power and abilities as a Proto-Zoalord were still useful to him at that point, so he was willing to give a little more leeway. However, his overarching need to have some modicum of control - if not complete control - over any situation that he finds himself involved in lead to more friction between him and Murakami than either of them would have preferred to deal with.
In the end, the two of them maintained an uneasy alliance, bound together by their mutual hatred of Chronos.
He does not enjoy the company of Ryan Crouger, seeing the younger boy as a random element in his plans that he would rather not have to deal with, but he tolerates him for the extra power he can bring to Agito's own war against Chronos. The fact that Ryan also seems to go out of his way to antagonize him does nothing to endear the younger Guyver to him, either.
Once he began to receive signals from another Guyver, Agito was quick to suggest to Ryan that he go and find the source of them. When the source of those signals was found - in the person of another Guyver named Sean Barker - Agito was quick to adjust his plans to accommodate both Sean and the faction of the US Army that was working with him.