Koichi Yamano (his family name differs depending on the anime version, including Furumi and Kamiya), also known as "Babel II", lives an ordinary junior high schooler's life, until one day when the computer of the Tower of Babel authorizes him as a master of the tower. Koichi is actually a distant descendant of the space alien Babel, who was cast ashore on Earth 5000 years ago and who tricked the people of Earth into building the legendary tower to signal to his friends where he had landed, only for one of the tower's computers to malfunction and explode, destroying the entire structure. Koichi inherits Babel's superhuman intellect, physical strength and other various supernatural powers. He leaves the Tower of Babel with three powerful servants following him; with their help, he confronts Yomi for disturbing the world's peace.

In the Giant Robo: Chikyuu ga Seishi Suru Hi OVA and manga series, Koichi is known as "Big Fire", the unquestioned leader of the organization of the same name. All of the Magnificent Ten serve him along with his three powerful servants, the shapeshifter Rodem/Achilles, the supersonic pterodactyl robot Ropross/Garuda, and the giant humanoid robot Poseidon/Neptune. Big Fire is highly mysterious, and we only get brief glimpses of him in the last few episodes of the OVA. He appears as a boy with silver hair and his powers are unknown, but they are suspected to be enormous.