Here is the history of Tobi.
Madara and Izuna loved eachother dearly, we know this. It is clear that Madara survived Hashirama because when Kabuto asked his first few questions to Edo Madara, that was one of them and Madara was suprised how much Kabuto knows.
So, moving forward, Madaras rival was Hashi. Izunas rival was Tobirama. Madara already said his brother willingly gave him his eyes, thus unlocking EMS. So now Izunas just an idiot with no eyes right? WRONG.
Madara and Izuna somehow (which clearly will be explained after the revelation) found Obitos dead body with a Sharigan that just lost its virginity...this was gold for eyeless Izuna. He took Obitos eye, and put on a mask to hide his identity. The mask makes sence because he only had ONE eye at the moment til he finds another! So with Madara dying, his Rinnegan needing a home, and Izuna with one eye and a mask, Madara came across
Nagato (somehow, which will be explained eventually) and decided (with Izuna) that he will give his eyes to Nagato for Izuna to safe guard and eventually take so Izuna can have a second eye ( which explains the new mask aside from Konan pwn). So Izuna, knowing Nagato will die because he is an UZUMAKI not an UCHIHA (which explains why Nagato was nasty and skinny) and cant have full controll of the eyes, began to make his move for the eyes, but little did he know Nagato believed in Naruto and that he was gunna get blown up for 10 minutes....so change of plans he had to kill Konan and force the eyes back. So now here is Izuna with both eyes back in their slots and ready to continue Infinite Tsukiyomi like he and his loving brother planned

of course i cant explain fully certain things, but the identity is tuere in our faces
SORRY FOR THE WALL OF TEXT BUT ITS WORTH IT
Ps to add on somewhere in there, i think Izuna may have stabbed Madara to unlock MS on the Obito eye...why else did Madara look inside his shirt before trying a Hashirama jutsu?