Gennosuke Yumi leads the Photon Power Laboratory as a scientist specializing in advanced robotics and energy research. He discovered Japanium ore beneath Mount Fuji, refining it to produce Photon Energy as a solution to global energy crises. He designed the Aphrodite A robot for peaceful purposes, later equipping it with defensive capabilities like the Oppai Missile System; his daughter, Sayaka Yumi, pilots it. He collaborates closely with Juzo Kabuto, creator of Mazinger Z, providing technical support. After Juzo Kabuto's assassination, he offers strategic guidance and stability to Koji Kabuto, Mazinger Z's pilot, against threats like Dr. Hell.

In the *Z Mazinger* storyline, he studies ancient Greek-like ruins on an artificial island he helped build. During an attack by Dr. Hell's Mechanical Beasts, he witnesses Koji Kabuto emerge with the body of the Olympian warrior Zeus. Gennosuke Yumi then rebuilds Zeus into the Z Mazinger robot for Koji, demonstrating his expertise in reverse-engineering ancient technology and focus on unraveling Olympian mysteries.

His personality shows measured wisdom and deep concern for his family. He demonstrates readiness for self-sacrifice during dire threats to the Photon Power Lab, reflecting a sense of duty akin to "a captain sinking with his ship." Though not overtly expressive, he shows clear care for Sayaka and effectively adopts Koji after Juzo's death.

In *Shin Mazinger Impact! Z Chapter*, he is known as Yanosuke Yumi. Here, he contributes to the Mazinger Army's robot designs alongside the "Three Doctors" and supports Kouji Kabuto's battles against Dr. Hell's forces, maintaining his role as a scientific authority while incorporating elements from other Go Nagai works.

His background before leading the Photon Power Lab remains unspecified across all media. The identity of Sayaka's mother is similarly undisclosed.