Who is Tobi Really??? He could be Madara's little brother (Izuna Uchiha) but he is suppose to be dead. He is the only one who is just as strong as Madara and he could of taken other uchiha member's eyes (since he gave his to his brother) like obito giving kakashi his eye. If i had to guess Tobi is Izuna Uchiha.
Tobi is somehow related to the Tobirama, the second hokage. I know he was summoned back by the edo tensei during the fight between Orochimaru and Hiruzen, but that means nothing considering the fact that Tobirama is the creator of edo tensei. Altough I doubt it is true, but he could have manipulated the jutsu in such a way that would allow him to be resurrected yet still have a body within the physical plane. As I aforementioned, I doubt this is the case but I stand firm with my opinion that Tobi has a great deal to do with Tobirama.
My reasoning for this is far more analytical than the conspicuous aspects such as the similarities in their name ("Tobi"rama); but the commonalities and mystery that encompasses Tobi are all traced back to Tobirama. Let us begin with Tobi's infiltration of Konoha's top secret and barrier placed around the village. Minato mentioned that this barrier was practical impenetrable and only a few people knew about it yet Tobi was able to slip through it undetected, obviously a previous kage would know how to successfully complete a task such as maneuvering through this objective undeteced. Moving on to the fight between Tobi and Minato, Minato mentioned that the space-time jutsu that Tobi possessed was equivalent to the space-time jutsu used by the second hokage Tobirama. Throughout the entire manga there are only three people that we've heard of that uses space-time jutsu, Minato, Tobirama, and Kakashi (with Obito's eye). We know Kakashi has one of Obito's eyes and the other got crushed by the boulder so it's not Obito. The only other person that is could possibly be is Tobirama, I also don't think that it is a mere coincidence that Kishi decided to, for the first time, mention that Tobirama could perform space-time jutsu during the fight between the most well known space-time jutsu user and the unknown space-time jutsu user.
Secondly, Let's examine the reappearance of Madara Uchiha via edo tensei. When he was resurrected, unlike everyone else, he knew exactly what was going on. He knew that he would be resurrected someday because it was a part of some plan that he concocted several years prior to his arrival. Obviously edo tensei is the key to this plan's success and when Madara was alive there was only one person that could perform the edo tensei, and that person is Tobirama. Kabuto stated that only three people could use the jutsu, himself, Orochimaru, and obviously the creator Tobi. Neither Kabuto nor Orochimaru were alive back then so Tobirama has to be a person that Madara created his plan to be resurrected with.
Thirdly, Tobi and Zetsu are obviously really close. Zetsu has never referred to Tobi as Madara nor has he directly called him that. He always calls Tobi, Tobi unlike most characters in the manga, why is that? Why is it that Tobi wanted people to believe that he was Madara but the person that knows him the best called him Tobi? To help you understand my point, a better question to ask yourself would be, Why was Tobi surprised and so easily persuaded to work with Kabuto once he saw that Kabuto had the ability to resurrect "the real" Madara Uchiha? Lets now examine the past activities of the Hashirama, Tobirama, and Madara. Hashirama and Madara fought to the death shortly after the village was created. Madara mentioned that he fought Hashirama to obtain his DNA. We know Madara used the DNA to obtain rinnegen but we also know that Zetsu was created using Hashirama's DNA. Shortly after the death of Hashirama, the first shinobi world war began. This is the war the Tobirama "supposedly" died. He "supposedly" knew that he would die when he fought against only 20 shinobi and appointed Hiruzen to be his successor. With only 20 enemies and the ability to perform space-time jutsu, it is highly unlikely that he would be killed in a fight like this, and even if he would have fell in this fight, there was certainly no way he would have known that he would die prior to engaging the enemy. If things did get bad for Tobirama in that fight he could have space-timed himself out of the fight and still been alive. Tobirama needed to fake his death in order to carry out the plan that he and Madara created. There plan was to create an immortal being out of Madara with the hopes of resurrected the real Madara one day to do the things that are taking place in the manga right now. Think about how easy it would be to put a mask on someone, give him a name, and say that he's been living for decades. It's like Batman, obviously Bruce Wayne won't always be Batman because he will die someday but he can easily train a younger person, give him the Batman suit, then the younger person will be Batman but the same idea of Batman is carried on into another generation, which creates the illusion that Batman is immortal. Tobi called himself Madara and done this "Batman" effect throughout the years to create the illusion that he is immortal. As a result, Madara knew he would be resurrected on day but he would not know who the person was that resurrected him, which is why he has to ask and found out that Kabuto was the person that performed edo tensei.
Lastly, Tobirama was Danou's teacher, he created Root, and he also appointed Danzou the leader of root. We know that Danzou and the root have been shrouded in mystery and they have been known to do unfathomable things in order to accomplish their goals. Since Tobirama was Danzou's sensei, he most likely taught Danzou about these covert tactics used to carry out missions and saw that Danzou excelled in this and appointed him the leader of Root. This doesn't necessarily prove that Tobirama is Tobi but it does help shine some light on the unkown past of Tobirama. We know absolutely nothing about his past which is odd since he was the Hokage. We know plenty about all the other kage yet Tobirama is the only one that we know nothing about.
I really don't see any loopholes in the this explanation and if you can find some please feel free to let me know.
