Monoshiri, known as the Wise Old Man, is a recurring character residing deep within Iga. A brilliant genius specializing in clockwork machines and inventions, he possesses a very short stature, often wearing high-heeled wooden geta to appear taller. A significant trait is his perverted nature; he frequently chases and looks at young women, collects pictures of them in magazines, and creates inventions aimed at aiding this pursuit. However, non-Japanese releases typically censor this aspect, replacing his magazines with car magazines and removing direct references to his interest in women, though the anime adaptations retain these habits. Despite this, he remains fundamentally good-natured.

As the series' inventor, Monoshiri creates numerous innovative, sometimes peculiar, devices central to the storyline. Key inventions include Sasuke, a clockwork robotic ninja initially guarding his castle who later becomes a playable ally; the Goemon Impact and Miss Impact, giant robots modeled after Goemon and Omitsu summoned by the Triton Shell to combat colossal threats; and a time machine originally intended to pursue women in the future. Other notable creations are the Super Gorgeous Ghost Machine for reviving the dead; a transporter moving characters between the game world and real world; the "Miracle Transport Machine," a fuse-operated cannon for travel; the "Cutie Detector" for identifying attractive women; Robo Tengu; and various other mechanical constructs.

Monoshiri aids the protagonists by providing critical inventions, guidance, or information. For instance, he directs them to use the White Mirror in Izumo to locate Princess Yuki and assists using his "Miracle Transport Machine." He operates from a palace guarded by robotic clockwork ninjas, requiring protagonists to navigate these defenses to reach him. His inventions often drive plot progression, enabling time travel or facilitating giant robot battles. While his perverted tendencies sometimes influence his creations' original purposes, these elements are downplayed in localized versions. His portrayal remains consistent as an eccentric inventor whose creations pivotally support the main characters' missions, with no significant evolution across different media beyond these established traits.