Gennai Ishikawa is a supporting character who appears as one of the survivors living in a self-contained dome called The Planet. Before the catastrophic event that turned Tokyo into a wasteland, it is hinted that he worked in the field of robotic research and artificial intelligence programming. He is a talented mechanic who puts his skills to use within the survivor community, primarily being in charge of the domes technical aspects.

In terms of appearance, Gennai is depicted as an old man with gray hair and black eyes. He is typically seen wearing a helmet with goggles, a rust-red scarf, and a striped shirt resembling a yukata, which features kanji characters on the chest. He completes this outfit with a black belt, black pants, and brown boots. His personality is characterized as quirky and intelligent, fitting the archetype of an eccentric but brilliant engineer.

One of Gennai Ishikawas most notable contributions to the story is the creation of No-sense, an artificial intelligent robot capable of human interaction. His mechanical expertise extends beyond initial construction, as he later modifies and upgrades No-sense. When the situation facing the group becomes more dire, he transforms the robot into a fighting machine, equipping it with weapons and tank tracks so it can join the diversion team in combat rather than simply serving as a guide.

Gennai shares a significant and close relationship with Foreman Oyakata Kurobe, fellow survivor and friend from the JAXA facility. Their bond is highlighted during a critical event known as the Wind of Death, when Gennai saves Kurobes life by giving him Haruto Kurosawas last aether. Kurobe later returns the favor, saving Gennai by running with him as fast as he could to the train station to escape the deadly phenomenon. This mutual rescue underscores a deep loyalty and friendship between the two men.