I started reading the original material, Musashi by Eiji Yoshikawa, a couple of months ago, and until where I'm at, the Sasaki Kojirou of the novel is a much better villain. Actually he doesn't even look as a villain in the manga.
Kojirou of the book met Musashi multiple times, talked to him and it was tense as hell. He watched the fight between him and the Yagyuu from afar and started talking shit about Musashi after that fight and spreading rumors about him being a murderer ( child murderer namely, as Musashi murdered the new young lord of the Yagyuu right when the fight began) not a samurai to anyone who sees, including Matahachi. But still has respect for him as a samurai and you can see the potential of a tense future fight between them. Novel Sasaki is a handsome and talented young prick.
And Matahachi, if you want this Matahachi dead then wait until you see the novel Matahachi. That guy is just too fucking dumb of a person you don't want to see him dead because death is too much for him. You want to see him die slowly.
Oh and as a reference, Kojirou had a monkey with him all the time in the novel, that's why he's called a monkey here.
The manga on its own is still pretty decent, there's still potential for some great confrontations and let alone the amazing quality of the art. And it's nice to see the series back after a long time.