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.