So ive just been thinking and i want to know how plausible these two theories are.
the manga revealed that it is indeed Obitos BODY behind the mask but it isnt 100% that it is Obito
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If Obito really is Tobi
If Tobi really is OBito then it seems almost impossible that he could control the Kyuubi at age 14. However during his fight with Minato, he was pelted with the rasengan and we saw that his arms substance seemed to be made from somethign zetsu like. Zetsu seems to be a fungus like guy so perhaps after the avalanche that "killed" Obito Zetsu protected Obitos body and basically used a white Zetsu to merge with Obitos body and in essence fix it. Somehow Obito found out zetsu has some connection with Senju DNA (thus showing him to enhance thhe white zetsus with more senju DNA aka Yamato) And that combined with the zetsu body mondifications gave him an enhanced Sharingan and enhanced strength speed and stamina. To awaken his MS perhaps he killed Rin, and then descovered the secrets of the Uchiha clan and then came into possession of Madaras, Izunas, or just a really strong sharingan for his other eye (or something to do with danzo since he had an asston of sharingans) and that is what gave him the ability to control the nine-tails.
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If Tobi isnt Obito
The person in Obitos body is a form of Madara or Izuna that manifested itself inside a zetsu form who then basically integrated into obitos body (maybe soemthing about another form of reanimation jutsu that merges witht he mosts body?) Since Obitos MS wasnt activated that person killed Rin obitos best friend thus awakening the MS and since the person already had a large amount of chakra he was able to control the ninetails by impleanting 1 more sharingan