Keita Aono is a seventeen-year-old high school student and the main human protagonist. A serious head injury in infancy required a transplant of dura mater from Laurier Noir. This procedure grants him exceptionally high readings as a Dual Kind, a special human capable of neural interfacing with advanced machinery.

A chance reunion with childhood friend Hinoki Sai leads to Keita's recruitment as a Head Diver for Akamatsu Industries. He pilots the Neuronoid mecha Kakuseijin to combat threats from the Algernon phenomenon, a mutation causing violent behavioral changes and physical transformations in victims. His advanced Dual Kind abilities, linked directly to his infant brain surgery, enable the rare feat of solo Neuronoid piloting.

Personality-wise, Keita contrasts with typical mecha protagonists. He initially appears awkward, prone to panic, and struggles helplessly in horror scenarios where conventional mecha combat fails. Despite this, he repeatedly demonstrates determination to protect others, especially Hinoki, whom he openly pursues romantically despite her frequent apathy. His development involves a shift from paralyzing fear to proactive sacrifice during crises.

Keita's involvement deepens as he investigates Algernon's origins with Akamatsu Industries and Mode Warp. His unique physiology and Dual Kind status make him key to confronting Algernon and the entity Kankel, though the titular Betterman often handles direct combat. His backstory intersects with broader mysteries like the Vivele Fruit and the fate of Hinoki's family, but his personal resolution remains focused on safeguarding those close to him.