Board Thread:Theories and Speculation/@comment-1783334-20150507053750/@comment-26038416-20150510051423

I honestly think it's either a someone new, who was working behind the scenes just like Obito and Madara were or it all ties back to Orochimaru. Shisui wouldn't make sense because if he faked his suicide then he would've saw what went down between Sasuke and Itachi thus there's no misunderstanding and he has no reason to just suddenly want revenge on Sasuke. Another suspicion that I think wouldn't fit is what some fans on the thread are saying it's Itachi's lover.

Itachi killed her along with the rest of the clan, Obito said it himself Itachi killed everyone in the clan, but he couldn't kill his little brother Sasuke. No Uchiha in the village during that event survived except Sasuke and during that time no Uchiha was going on missions except Itachi and Shisui, so there was no Uchiha who was gone on a mission when the massacre happened and even after that, it was confirmed that Sasuke and Itachi was the last Uchiha's because at the time no one knew about Obito.