Dr. Kotaro Azuma designed BK-1, a pollution control robot meant to aid humanity. After a lightning strike granted it self-awareness, the robot rebelled under the name Buraiking Boss (or Black King-1), creating an army to overthrow humans, whom it deemed destroyers of Earth's ecosystem.

In the 1973 storyline, Azuma transformed his son Tetsuya into the cyborg Casshan to fight the robots. Captured alongside his wife Midori by the Android Army, he was forced to develop anti-Casshan weapons like the robot Ace. He later escaped, crafted the Sprazer weapon system to annihilate the Android Army, and reunited with Tetsuya.

The 1993 OVA reimagines Azuma’s fate: Black King-1 kills him and Midori, then imprisons digital copies of their consciousness—Azuma’s within its own systems, forcing him to witness its actions, and Midori’s within a robotic swan.

The 2004 live-action film portrays Azuma as a geneticist developing "neo-cell" technology to cure Midori’s blindness and heal war-related biological damage. After their son Tetsuya dies in combat, an electrical storm activates his experimental tissue tanks, spawning mutant Neo-Sapiens who abduct Midori. He battles both military forces and the Neo-Sapiens while confronting his research’s fallout.

His role in the 2008 *Casshern Sins* reboot remains undocumented.