Tobi is either Kagami or Izuna. Both of them were alive during the reign of Madara and Hashirama. I am leaning more towards Izuna because Izuna resembled Madara A LOT.
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Izuna:
If Kabuto was able to rob Madara's grave for his DNA, what's stopping him from getting Izuna's DNA? Izuna was as strong as Madara when he was alive so why wouldn't Kabuto revive him? He gave a reasoning as to why Shisui and Jiraiya (both extremely powerful) were not revived but he never mentions ANYTHING about Izuna.
I am going to throw a random theory out there.
Izuna did not die. Madara took Izuna's eyes in order to rival Hashirama's overwhelming power. Izuna uses replacement eyes. He is a Uchiha in the sense of blood and body so the eye does not cause strain on him but it has its downsides. One of them being the inability to use use MS.
Out of jealousy, anger and hatred, Madara challenged Hashirama to a death-match in order to gain Rinnegan and Hashirama's powers. Izuna and Madara predicted the death of Madara. Madara obtained the Rinnegan but died. Izuna took the Rinnegan and gave it to Nagato and started working on the Moon's Eye plan. In the future, Izuna would revive Madara using the Outer Path ability.
The abilities he achieved overtime could be the result of ganking Hashirama and Tobirama. Furthermore, Tobirama would likely Izuna's rival like their older brothers. This explains how he could have obtained his Space-Time technique but it does not explain his lack of knowledge on Edo Tensei.
NO ONE seems to acknowledge Izuna. Even Minato thought it was Madara.
Kagami:
Could have been Madara's pupil as Tobirama is clearly seen leading 2 teams in the war. Explains how Tobi would know both the elders, Danzo, Hiruzen. We don't know what happened to him.
He was also under Tobirama's command. Who's to say he wasn't trained by Tobirama?
Kagami is less likely because there's just not that much info on him.