After One stabs Zero, his white clothing is turned red by her blood. His appearance after this is likely a reference to Manah, who wore a red ceremonial robe in Drakengard.
His face bears great resemblance to Manah and Seere. It is most likely that One and her twin brother are a reference to their younger counterparts:
Both Manah and One are the heads of a religious group.
Seere and Brother One are trying to save their sisters.
Both Seere and Manah are direct descendants of Brother One. Their mother Ramia is One's great-granddaughter.
It is unclear if One's decapitated head is still alive in some way. He was either beheaded with his Dragonbone Sword (which should instantly kill him), or he was killed with regular blade (which would explain the magic circle on his neck.)
He is the narrator in the Drag-On Dragoon 3 Story Side novel.
The symbol of the Cult of the Watchers from the first two games actually represents One and her twin brother, making it a sort of early cameo before their debut in the third game, although this was obviously made in retrospect.