Agul's name was only revealed in episode 16. Prior to that, he was simply called Blue Giant or Blue Ultraman. The name first came out from the romaji "AGUL" displayed on the console screen of Fujimiya's personal computer. The word itself is a coined word, coming from the English words "aggressive" and "agreement," as well as the Japanese word aku.

Agul's human host, Hiroya Fujimiya, a boy-genius formerly with Alchemy Stars. Fujimiya created the giant computer known as "Crisis," to predict future catastrophes. Its first prediction, however, showed the total destruction of the Earth. When queried further, the computer had one response: AGUL. At that moment, Fujimiya yelled in pain. In a vision, he saw a destroyed world, with a blue-and-silver giant towering over it. When questioning Crisis again, Fujimiya discovered the cause of Earth's destruction: mankind.

Fujimiya breaks away from Alchemy Stars, obsessed with discovering more about this being that continues to contact him, although each contact brings great pain. He builds a giant water tank, the basis of an experiment to contact "the will of the Earth" to find out what to do. At one point, as Fujimiya stands on a catwalk above the tank, Agul contacts him again. This time, Fujimiya sees the surroundings: complete destruction, the remains of a civilization similar to Earth's. In shock, Fujimiya tries to shut down the experiment, but is convinced by his friend Kyoko Inamori to keep going. Returning to the tank, Fujimiya witnesses a great light forming at the bottom of the underwater tunnel. The light erupts from the tank, and flows into Fujimiya.

After this, Fujimiya shuns all human contact, although he still has feelings for Kyoko, and develops feelings for a female reporter; however, when she doubts Agul's benefit, Fujimiya shows resentment.