Itachi Uchiha, Sasuke’s “evil” older brother while Byakuya is Rukia’s “evil” “brother”. While Itachi is a formidable foe seeing as how he hasn’t lost yet Byakuya has…BANKAI. With it he’s almost unstoppable. Byakuya is also the 2nd strongest Bleach character ever. Byakuya wins.
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Byakuya has his Zanpaktou but Itachi has susanoo in which he won’t be affected by anything. Itachi also has Amaterasu and Tsukuyomi. Itachi wins!
Itachi is definitely one of the strongest Naruto characters, but he still doesn’t win against Byakuya. Susanoo is a tough attack, but it can’t be maintained for too long given Itachi’s eyes are going bad. Byakuya could last long enough, and with his speed he could get past Susanoo and strike a fatal blow