At this current point and time unless Kishi screws up his own info I am absolutely confident that behind Tobis mask is Kagami.
Let me elaborate:
First of all:
Why did he go by the name of Madara?
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"Why did the masked man claim that he is Madara?"
"Madara is more than a man, he is an Ideal."
That info is highly important as we were informed about something that shows just how much power the name as well as more info on why taking his Ideals to hear.
I present this:
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It was Nidaime who screwed them over, lying about his sign of trust as it was merely a method to surveille them as well as distancing the Uchiha from the village.Ultimately I point to the 3rd Panel.
"Some of the Uchiha realized what was going on.Some Rebels
began to follow in my footsteps."
I bring your attention to the stone monument behind them having Madara written on it.
And consider this as well: Why did he chose the name "Tobi"?
Evidence for Kagami?
There are several indications Kagami was among the Rebels, most namely by looking at some panels from the past and present.
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"Long time no see Danzo...since the Uchiha Incident isn't it?"
Uchiha Incident is the key here.One would immediately think of the Massacre but matter of fact is there were 2.
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The Kyuubi Incident where the Uchiha Leaders suspected an Uchiha behind the Incident.Naturally they were right however I bring your attention to this:
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The elders suspected an Uchiha and who are the two ones from their Gen not fighting alongside them?Kagami and Danzo.
We know Danzo and Tobi met during a Kyuubi Incident and incidentally the Elders suspecting an Uchiha makes absolute sense this way.
Two of their Teammates were missing.One being an Uchiha.
And I bring attention to this:
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Kushina herself stated that the info of the Birth was highly secret with few elders knowing about it.I once again point to Tobi somehow gathering knowledge about the birth.So yes:
The Elders suspected Kagami because he was missing.
At the same time Danzo sold out the info to Kagami.
One can also see a pattern of how Kagami fits into the Uchiha Massacre.
Consider the ones who are behind it with the Elders.
Homura,Koharu,Hiruzen and Danzo.
Add Kagami to the bunch and you have nothing else but 5 of his last team being involved in their downfall.
And this brings me to Torifu.
We were shown he survived the Kyuubi Incident yet for some reason his status past that is unknown.
Meanwhile we know Tobi played some role in the Night of the Uchihas massacre.
And for a matter of fact, Tobi had a different mask back then than his two last ones.
In other words there must be a reason for Tobi to have changed his mask as we know he only changed the last one due to it being destroyed.
I propose this:
Tobi's reason of mask changed was that it got destroyed on that very Night.
Incidentally the absence of Torifu despite showing his survival is highly suspicious even for a Minor Character.
So yes on that very night:
Torifu and Tobi engaged in a fight resulting in the latters mask being shattered.
However for Torifu he was welcomed by a face he knew, that being the face of his friend Kagami who silenced his comrade.
The set is complete.The reason for the new mask given.And we can see a majorly pattern.
Homura,Koharu,Torifu,Kagami,Hiruzen,Danzo.
All 6 people playing a important role in the massacre of the Uchiha.
Incidentally, I note to the masked man using the name TOBI.
Matter of fact this pattern leaves us with my final points:
Who was the one that caused the Uchiha to began rebelling thus possibly resulting in the birth of Tobi and the entire mess we know thus far?
It was,Nidaime, TOBIrama Senju.
Who are the ones the sealed the very fate of these very Uchiha who planned a Coup and are the key figures if my theory is right?
Koharu,Hiruzen,Homura,Kagami,Danzo and Torifu.
Incidentally the last Team of Tobirama who caused this mess.
My final thought:
Kagami had become one of the Uchiha Rebels due to Tobirama screwing them over ultimately becoming Madaras ally.
The reason he chose the name Tobi is out of resentment towards his former friend, the reason he chose the name Madara was to continue on his legacy and the Moons Eye Plan.
This explains his hatred towards both Senju and Uchiha.
Madara hated the Senju due to their surpremacy but hated his clan for turning their backs on him when he needed them most.
After seeing what his friend Tobirama had done, he saw Madaras Ideals as right and began resenting his own clan as well as if they would have followed Madara the Surpremacy of the Senju wouldn't even exist.
After the death of the real Madara became Madara himself.
However due to this he needed someone to inject the Hashirama Cells into his body which is where Orochimaru came in.
This marked the day where Orochimaru got his hands on Hashiramas DNA thus beginning his experiments on the Children with Yamato surviving.