Mashumaro Ujuin (宇集院 魔朱麿) is the son of Ms. Ujuin. He comes from a very wealthy family and his home is the most luxurious in Kasukabe. He goes to the Futaba Kindergarten and has a dog which he loves a lot. He appeared in manga volume 9, and in anime episode 114-3 "The parents interfere with the children fight" (子供のケンカに親が出るゾ).
He reappeared on the episode 574-2, where he was brought to the free classes that an English academy was offering, and he met Shin-chan there.