(from ebookjapan):

Seitoku Juen, the son of the bathhouse owner, is the naughtiest and the most mischievous boy in his class. He screws up everything he does, and his well-meaning actions always backfire. His parents are angry at him for his poor grades, and the general kids bully him...

However, he has a pure heart and will not tolerate anything crooked. He wants to be a great big brother for his cute little sister, Taishi, but what will he do?