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Agito Makishima
Agito Makishima was the adopted son of the branch chief of Chronos Japan. Through his own machinations, Makishima was able to get his hands on the third Guyver Unit, and discovered many of its abilities. He maintained his contacts and position in Chronos until his treachery was discovered and he and Sho destroyed Chronos Japan. Makishima has his own motives which go much deeper than either Sho or Tetsuro have guessed.
Mizuki has (or had?) a crush on Agito from issue #1, a fact which caused Sho no end of grief. Of course, Sho had no idea that Makashima had anything to do with Chronos or that Agito was Guyver III. 
Sho had met Guyver III many times, but it was not until after the fall of Chronos Japan that Sho knew the truth. Makishima rescued Mizuki and Tetsuro while the Guyver Unit was regenerating Sho's body. Then, after Sho was awakened from his coma, he and sho destroyed Chronos Japan and the zoanoid research facility there. 
Makishima also took Sho, Mizuki, and Tetsuro to a house that he had kept near Relics Point for just that reason. They stayed there until they were found out, after which they were transferred to yet another secure location.
Agito is more cold and ruthless than Sho is, and that, combined with his superior fighting ability, makes him in many ways a more dangerous adversary. His Bio-Booster Armor is also cosmetically different than that of Guyver I or Guyver II, which makes him a more striking figure.
Agito is a bizarre character, whose motives are hard to decipher. It's not quite clear where he stands, but it seems as though he is waging his own war, working with Sho only because he is the host for Guyver I.