Manzo Momoi is the grandfather of Nanako Momoi in Boku to Kanojo no XXX. He is an eccentric and somewhat senile inventor who is deeply devoted to his scientific experiments, which he pursues with the stated goal of benefiting the planet. His most notable creation is a machine capable of swapping the consciousnesses of two people. This invention is what sets the story in motion when he accidentally uses it on his granddaughter Nanako and her classmate Akira Uehara, causing them to exchange bodies. Manzo Momoi appears as the catalyst for the central body-swap premise. His personality is characterized by a single-minded, almost reckless enthusiasm for his work, with little concern for the consequences his experiments cause for those around him. After the initial swap, he later disappears on a secret trip, leaving the main characters unable to reverse the swap and forcing them to adapt to their new situation. His key relationship is with his granddaughter Nanako, though he also interacts with Akira as an unwilling test subject. He shows no significant character development over the course of the story, remaining a mostly static figure whose inventions drive the plot forward. His abilities are limited to his scientific ingenuity, particularly in the creation of experimental devices.