After the losing to Hashirama, Madara gained a portion of his strength. This soon led him to awaken the Rinnegan. Madara stated that he could have "taught Hashirama how to become immortal", very much meaning that he meant he could have given Hashirama the Rinnegan also, since Senju + Uchiha creates the Rinnegan.
The Rinnegan is known for its "god" abilities, those used by the Sage of Six Paths. The SOSP was able to split entities such as the Ten Tails into different beings, the tailed beasts. Madara could very much do the same to himself, whether using the Rinnegan combined with some kind of Izanagi technique, to split a portion of his soul/mind onto the Shinobi world, and then he would die.
The soul soon entered the Third Shinobi War, trying to find a host body to maintain itself. Thus, it soon found young Obito, near death or just a corpse and was possessed by Madara. This may be odd since there were many hosts years on later after Itachi massacred his entire clan, but he couldn't have possibly known that and just wanted a body ASAP. In any case Madara was alive again, in a body of another Uchiha. The reason why I suggest it's Obito's body is because of Tobi's eye. It doesn't make sense why he would use that one, the only logical possibility is that it was already on his body. Otherwise he would have to search MANY sharingan eyes to find a good one. Tobi also mentioned of showing Kakashi the "real Kamui", suggesting that Tobi's body is Obito's.
Tobi has also mentioned that he is only a "shell of his former self", this very much gives enough evidence that he stole someone's body. Obito's body seems about right, same hair, it's body damaged. Obito's body was crushed so I'm guessing he'd probably use Zetsu to replace his other body parts and this also explains his growth in height during when Kakashi was in his teens. Although Kakashi was only around 14-16 so not that short during the period of the Nine-tails attack on Konoha.
Tobi's name, is also similar to Obito's, just rearranged. This could be because Madara still needed time to act in secret and to form the Akatsuki so not to get noticed. Tobi has also said "I am no one, I don't want to be anyone, my only goal is the Moon Eye Plan", this makes sense if it was a portion of Madara's soul because Tobi would be made of up so many people that he wouldn't even suggest he was a person anymore. I'd rule out Izuna because his eyes was stolen by Madara, so why would he ever help him?
I'm 95% sure with everything I've said. The reason for being this confident is that I also theorized Tobi had MS, which no one believed me xd, but that was an easy guess. We'll find out next time, otherwise we're being trolled lol.
No violent comments please*** XD
