Mokubei is a minor character in Mystical Ninja Starring Goemon. He went to Mt. Fuji and set a temporary home there so that he "could train to become the greatest worker in Japan". He owns a big collection of pipes, which he presumably crafted himself.
When Goemon meets him at the top of the mountain, he lends him his Chain Pipe, a retractile chained pipe that can attach itself to distant blocks bearing a star mark, allowing the player to hop across large gaps.