There is speculation about Tobi being that Kagami Uchiha kid. But personally, I think we won't find out until much later in the series and a little more info can be squeezed out of anime/manga.
We can say right now is that the this little bas tard is NOT human. Due to the whole white blood stuff comin' out him in that one chapter . . . forget where it is . . . It was during the Konan fight, right? We MUST keep that in mind though.
Okay, there are two reasonable answers of Tobi's identity.
It CAN only be someone who has many years of experience and old age. When you see Tobi take his mask off when Zetsu was with him you see he has liver spots, wrinkles, ect. So it CANT be Obito. But it MAY be true that Madara found Obito and used his body by using Zetsu's first Hokage cells. Another reason why it CANT be Obito is the lack of motivation of going bad. Why would a good, kind hearted, heck of a ninja just go bad??? It MAY be true that it is indeed Obito's body but Madara. You may complain that Tobi has the same sharingan power as Kakashi, which has Obito's sharingan,but Madara and Izuna were the strongest of the Uchia, don't you think that they would already master a amateur sharingan of a 12-13 year old boy???
Madara Uchiha is one reasonable candidate because of his experience and knowledge of the ninja world. When Tobi was fighting Minato, Minato noticed that the "masked man" can control the nine tails. The battle between Madara and Hashirama is already out, you see him summon nine-tails. Only the most elite of Uchiha can do this, which is only Madara and Izuna. The only person that was on Madara's level was his twin brother Izuna.
Remember, Madara took Izuna's eyes, later on "Tobi" declares when he was fighting Konan that he was just taking back what was his, this basically meaning that the true identity of Tobi must be a descendant of the Sage of Six Paths, the true wielder of the Rinnengan. If you read the manga Madara unlocked the power of the Rinnengan after he fought Hashirama. Escaping with Hashirama's DNA. Therefore creating Zetsu. He could have used Zetsu's special ability to form himself with Obito's corpse. Which is why it MIGHT be true that Tobi is a part of Madara Uchiha and Zetsu together.Remember Tobi, or Madara declared that he was just a shadow of his former self, that he was incomplete. This meaning he is not whole! Which is very LIKELY why Tobi and Madara are at two different spots. When Kabutomaru showed what was in the coffin Tobi was really interested something that was VERY important to him. If you think its Madara it could be his true body, so he can be whole. If you believe its Izuna, the one in the coffin can be his brother, Madara, which is already identified that it was indeed Madara in it, in the latest chapter.
Tobi CAN be Izuna because, as I said, when Tobi was fighting Konan he declared he's just taking back whats his. Madara did take Izuna's eyes. When Madara unlocked the Rinnengan he gave it to Nagato, convincing him to make the Akatsuki. MAYBE Izuna's trying to get his eyes back. Madara and Izuna were always known as equals and even in EVERYWAY meaning it is VERY POSSIBLE that they have the same abilities.
IM NOT SURE WHO IT IS. BUT I WANT TO KNOW RIGHT NOW!!!!!!!!!!!!!!!!>:3
MY GUESS, HE'S MADARA UCHIHA
I've finished my research Tobi is a clone of Zetsu that has Madara's soul sealed inside. That is why when he is injured he be oozing that white stuff. Thats why he keeps saying that he is not whole and just a show of his former self. This is the reason he can use Izanagi, a techinque that is needed for a person that is a Senju and Uchiha, Zetsu has Hashirama Senju's DNA. So this is why Tobi is indeed Madara. This is why Madara's body was ressurected by Kabutomaru.
You may be wondering why the character's name is Tobi. Well, i did more search and it turns out he is named after one of the staff who helped make the naruto series.
I am 100% sure Tobi is Izuna Uchiha here are three reasons
1)It is revealed that Tobi must be of old age which is why it can't be Obito or anyone around that generation
2)If Izuna Uchiha was second to Madara why hasn't Kabuto used Edo Tensei to bring him back...because he can't since Izuna(Tobi) is not dead.
3)Right before Madara dies he unlocks the Rinnegan. Have you ever wondered what happened to Madara's eyes when he died? They were implanted to Nagato and so forth. Before Tobi fights with Konan he clearly states to Konan that the Rinnegan is not Nagato's it's he's(Tobi). When he said that I just knew who it was. Think about it the Rinnegan Nagato had was actually Madara's eyes. Tobi says its his eyes because Madara took his younger brother's sharingan and whose his younger brother IZUNA UCHIHA.
If its not Izuna im gonna fcking laugh at you smile)). Izuna got his eyes ripped out HOW is he tobi who has 2 eyes? Huh? Madara TOOK the eyes himself and was WITH Izuna when he died. And WHY would MADARA LIE about his BROTHERS DEATH? Hmm?
This goes for everyone that believes what Tobi says. Remember, this is the same guy that's been telling everyone that he was Madara. He lied about that. Remember, everything he says always has a chance of being a lie. We have proof that he has lied before, he could always lie again.
Things I believe that Tobi has possibly lied about:
1. His name (we already know about this)
2. His story to Sasuke about Itachi's past. It's very possible that he left some parts out to purposely manipulate Sasuke. An example is if Itachi willingly chose to undertake the clan extermination mission to make sure he could save Sasuke. Obviously it is easier to get Sasuke to be against the leaf village if he says "Yeah, the village forced him to do it" instead of "Yeah, Itachi chose the mission willingly so that he could make sure he saved your life Sasuke"
3. I feel this is the most obvious.
Itachi stated that Tobi forcefully took his brother's eyes for himself.
Tobi stated that Madara's brother willing gave up his eyes for his older brother.
Yeah........its kinda obvious. Madara probably did steal his brother's eyes.
Then again for Izuna supporters, this could work in the other way. If Tobi really is Izuna, it would make sense for him to correct Itachi, as it hints about his feelings for Madara. Feelings a little brother would have toward his older brother.
4. If Izuna is truly dead or not.
Its not connected to anything specifically. What I mean by that is, if its a lie, or if its true, it doesn't affect anything except the chance that Tobi actually could be Izuna.
As for the Izuna theory, lots of stuff adds up about him. The only solid thing stopping it is that Tobi, A KNOWN LIAR, stated that Izuna died.
What we really just need is a flashback from Madara himself. That would pretty much solve everything.