Satan

Satan is the main antagonist of Blue Exorcist, ruler of the demon world Gehenna and father of Rin, Yukio Okumura, and the Eight Demon Kings, including Mephisto Pheles and Amaimon. His existence began as an ego-less spark of blue light known as a Rinka, worshipped and feared under many names across cultures before fully forming a consciousness only a few years before the Blue Night.

Lacking any real moral framework in his earliest years, Satan viewed the world with a childlike, amoral curiosity, later hardening into cruelty and megalomania. He created Rin, by his own account, almost out of boredom, only to later recognize his son as the key to conquering the human world of Assiah, since Rin uniquely carries Satan's power while being born in Assiah itself.

Satan's unhinged laughter and casual cruelty extend even to his own children, and his sole documented human connection was with the exorcist Yuri Egin, whose faith in his redemption never fully softened the destruction he wrought during the Blue Night.