Hosaku Kamon (加門 豊作) is a kid than went to the Futaba Kindergarten (particularly to the Rose class) as an exchange student during the summer holidays. His biggest hobby is gardening and he is an expert detecting potatoes underground. He appeared on the episode 581-2 (イモ掘り対決だゾ) of the anime and in the 44th volume of the manga.
All the kids go to the countryside to pick up potatoes. The kids take it as a competition between classes, and Shinnosuke and his friends had nothing to do against the Rose class, as Hosaku was leading by far getting the biggest potatoes. He is also the only one to discover that Ume Matsuzaka is not the city woman she pretends to be, based on how she behaves in the field.
Finally, he is defeat as Shin-chan and his friends find an extremely huge potato, 'one every one hundred years', according to Hosaku who feels completely destroyed.