Iwanbou is a fat oaf who uses his immense size, his strength, and his ability to entrap weapons in his flab. He is very stupid, and, apart from taking part in the battle of Aoiya as well as grinning like an utter idiot, he doesn't say anything, and he doesn't do anything. He was last seen on a cliff outside of Kyoto.

As revealed in the Jinchūuu or "Revenge" arc of the manga, Iwanbou is in reality a mechanical puppet, a primitive mech, if you will, controlled by Gein, the puppetmaster. Gein reappears in this arc as one of the six comrades in a more dangerous form of Iwanbo and plays a major role in Enishi's revenge.

(Source: Wikipedia)