Satō Seiza was a past Atsuko Leader.
Satō Seiza was the only Atsuko Leader (Urshanabi) out of three who survived the Second World War. In the absence of the fallen leaders, two more Atsuko were raised: Heran Otsul and Akari Nakano. However, because of their rejection of Satō's proposal of a unifying war against the other clans, the Betrayer murdered both of them. With total control, he was able to coerce and threaten the Atsuko people into what would become known as the Black Clan War.
In later findings, it was found that Satō believed in frightening visions that only he was able to see. Over the course of several journal inscriptions, it would seem his hold on reality had slipped further and further until the end of the war.
His secret mission in invading the other clans was to gain possession of both the Aramar Shard and Daisuke Shard. Thwarted twice by the wisdom of the other clan leaders, the Betrayer was never able to accomplish his goal.
Over two long years of searching and fighting, his army grew strained, ultimately leading to their fall. On the Day of Long Death, Orin Aramar initiated a trap and with the allied forces of the Aramar and Daisuke, defeated Satō Seiza's army, ending the war.
At the end of the battle, Orin Aramar held his blade to Satō Seiza's neck and struck, but was deflected by Manaia Hahona—the leader of the Daisuke—who spoke the famous words, "There has been enough blood spilt in Ninava. Let mine be the last."
Because of Manaia's mercy, Satō Seiza was able to live out the rest of his life in imprisonment.