Volume 16 contains the side story "Natsu no Leisure" (夏のレジャー).
Pyuu to Fuku! Jaguar (ピューと吹く! ジャガー)
A special that was written by Masuda Kousuke, the mangaka of Gag Mangabiyori.
Poo (ポー); 2010 #48
The Chiu nation is a growing nation of martial art fighters. Literally hundreds of styles of combat with thousands of fighters are on a constant grind to master their skills. Ren Hogin is a student of the Hakkesho style, a style in which he creates hand movements that induce the enemy to throw up.
Ren meets Poo Chomkin, son of the world famous fighter, Poo Kubitz, who was ridiculed for his incredibly stupid style, the Funshimuso style, which was never even accepted as a real style. In actuality, the Funshimuso style was such a ground breaking style that the institution smeared it in order to get rid of it. The father disappeared after finishing the ultimate style and has not been heard from since. Now, the combat genius, Poo Chomkin, is on a mission to find his father and rediscover the style. However, there are powerful dark forces that will stop at nothing to drown the reemergence of the the Funshimuso style.