Akira Asagumo is the son of Shigeru Asagumo and grandson of Dr. Asagumo, a scientist involved in the Nazi-led Big X project during World War II. To conceal Big X's secret, Dr. Asagumo implanted a card inscribed with its formula into Shigeru's body. Twenty years later, surgeons discovered this card during an operation on Shigeru in Tokyo. After the Nazi Alliance stole and completed the Big X project, Akira recovered the formula.

Big X is a drug enabling unlimited expansion of the human body. Akira uses this drug to transform into a giant-sized superhero, adopting the name Big X. In his transformed state, he wears a distinctive scarlet costume with a winged helmet. He combats the Nazi Alliance, particularly Hans Engel (grandson of Dr. Asagumo's co-researcher, Dr. Engel), who seeks world domination. The transformation mechanism differed: the manga depicted an injection, while the anime adaptation used a magical amulet due to censorship concerns regarding drug references.

Akira dedicates himself to challenging the Nazi Alliance and thwarting their plans. Beyond his primary series, the character appeared as "a Champion of Justice" in a 1965 Astro Boy story, had a brief appearance in Gum Gum Punch (1969), a cameo in Gachaboi’s Biography (1970), and non-speaking roles as a spectator in Rubbish Poetry (1969) and Black Jack (1973). He also appeared in the video game Astro Boy: Omega Factor (2004) alongside other characters created by Osamu Tezuka.