@ Obsidian Fire: Doesn't Orochimaru attempt to bring back 4th Hokage during the first with the 3rd, but the 3rd blocks the seal? I remember that from the anime, when he brings up the two coffins (marked 1 & 2), and gets the other (marked 4) half way up, but it gets pushed back down. I can't find the manga chapter, however.
On a different note:
So, it's theory time again. I was thinking about Tobi as I read back through ~370-400, and I think that I've got him figured out, and it ties VERY closely in to current events.
Zetsu hasn't been fully explained, it got me thinking about why he's there, and what he has to offer. Orochimaru left Akatsuki 10 years ago. Of the known members, he was the one who primarily experimented on the 1st Hokage's cells. Yamato was the only one of 60 subjects who survived, and Danzou was hardly perfect. That means that normally it's an incredibly risky procedure, and it can go wrong, like it did with Danzou. The only person who is able to successfully posses and use the First Hokage's cells aside from Yamato in any way is Zetsu. While not a direct integrations, he is able to integrate with his other half, created from the 1st Hokage's cells.
Zetsu has the habit of talking of even having thought conversations with "himself" between his dark, and his light half, suggesting that they are entirely unique personas. Tobi was introduced as Zetsu's pupil, which in a sense may be true if Zetsu was responsible for Tobi's creation on Madara's behalf to have the first's cells and the Sharingan. We know that Zetsu and Madara have been working together with the 1st Hokage's cells, it makes sense that Zetsu is the one who allowed Madara to successfully merge with those cells.
If the process was anything similiar to how it works for Zetsu, it's likely that Tobi was the manifestation of a completely unique newer/younger/goofier persona generated by the 1st Hokage's cells. Until Madara gained some semblance of control over it, he couldn't be an active member of Akatsuki. This would also match with Itachi's observation that he's a shadow of his former self. As he becomes more active in Akatsuki, we slowly see his Madara emerging more completely, and gaining full control over the 1st's cells' personality - Tobi. and he "becomes" Madara. This would also make sense as to why Zetsu still calls Madara "Tobi" during their conversations.
It should also be noted that we've ALMOST never seen the left side of Madara's face. In chapter 510, on Page 8, 10, & 16, after he breaks off the right side of his mask to save himself, and then speaks with Konan. This half of his face seems covered in Shadow, but his skin is SUPER dark even for being shadowed, and it's also pitch black when viewed from above. This is also the opposite side of Zetsu's Black half. If so, this would explain where Zetsu's other half is, and what it's being used for, in addition to who Tobi was/is, and what Madara's relationship with Zetsu is. This would also explain his need to constantly wear a mask.
The only question is - now that they've got Yamato, what implications does this have for Madara & Zetsu...
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