Historical aspects of IZO

I have been reading numerous posts on who IZO is supposed to represent. The funny thing is that not once have I seen any reference to the fact that it is based on real people. Here is some info.

Okada Izo; (1832 – June 3, 1865), born Yoshifuru, was one of the four members of the Hitokiri, a band of assassins active in Japan during the late Tokugawa shogunate in the 1860s. He and Tanaka Shimbe worked for Takechi Hanpeita as political assassins attempting to kill everyone who supported the Tokugawa Shogun over the Emperor. Okada started out as a poor uneducated peasant, but because of his skill and his strong fighting spirit he quickly distinguished himself, becoming one of the most feared and renowned Hitokiri. He even has a sword that can still be purchased today named after him. It is the Jidai Koshirae Izo.

That was all I could really find on him but I really didn't look long. If anyone else finds anything else that is historically relevant please post.