Suggested by Destroyer Sigma is an incredibly powerful Megaman X villain. He has had many different forms over the years and an impressive skillset. That being said, he isn’t quite ready to deal with Byakuya. Byakuya is one of the strongest captains in Bleach with impressive speed and power that go completely beyond what the average fighter can hope to deal with. Sigma can fight hard but he definitely will not be ready to take down this guy 1 on 1. Byakuya wins.