Manji

He's known infamously both as the "Hundred Man Killer" for the number of men he's killed by the beginning of the series, and "Mister Twelve Blades" for the number of weapons he carries. He is most easily identified by the manji symbol he wears from which he derives his name and the scars across his face, and as the title of the manga implies, he's been given immortality by the means of the Kessen-chu, which prevent him (for the most part) from dying.

Little is known about Manji's background except that he once served under a local daimyo, unknowingly enforcing his brutal rule on the locals until he found out his lord was corrupt and killed him. For that, he was hunted as an outlaw. He was later nicknamed "Killer of 100" after the number of police officers he killed.