I'm gonna have to say Shisui Uchiha. Used Kotoamatsukami on Ao's Division which suggests they encountered each other at the peak of the 3rd War.(Kyuubi attack happened shortly after the war, Itachi was only 7)
So he had to be like 8-10 when using Mangekyou
If he grew to be 18 or older, he'd surely make even Madara shake in his ninja sandles lol
So he had to be like 8-10 when using Mangekyou
If he grew to be 18 or older, he'd surely make even Madara shake in his ninja sandles lol