Guy: "Just... Who are you...?!"
Tobi: "You don't remember faces, so what point is there in telling you ?" <--- Translation: What's the point in telling you who I am when you didn't even remember who Kisame was the second time you fought him ? The next day you would forget who I am... <--- Tobi was being ironic to Guy sensei !
People I was also fooled the first time I read what Tobi said.
I think Tobi is Kagami Uchiha, he might have been a teacher at the academy when Kakashi and Might Guy were just Genins, the Izuna theory gets thrown out the window cuz Madara even said he's dead and Tobi doesn't have his Mangekyou Sharingan and he's was born and died YEARS before Kakashi and Might Guy were born, it can't be Obito cuz he doesn't know much and if it was him he would have known more about Minato cuz he was shocked when Minato whooped him