Seikyo

Seikyo (逝去, "Demise"), also known as the King of Oblivion (忘却の王, "Bōkyaku no Ō") and the Envoy of Destruction (破壊の使臣, "Hakai no Shishin") is a being who is rumoured to have existed before the time of Soul Society itself. Very little is known about Seikyo, except that it now exists within the soul of Gai Nagareboshi as his "Awakened Form".

In truth, Seikyo was a king of an unspecified spiritual realm, who led a campaign to obtain the powers of the Head Seishin and Soul King, destroying everyone and everything in his path to get it. In retaliation, Nanaya Wanijima and Kazuma Nagareboshi teamed up to defeat Seikyo, with Kazuma dealing a mortal blow that reverted Seikyo to a soul. After, Seikyo's soul scattered to the winds, losing all memories, before reanimating a stillborn Gai Nagareboshi, assuming Gai's identity. Seikyo went into remission in this time, and the new constructed personality Gai took the forefront.

Due to Seikyo's presence being nearly faded, not a single person when Kazuma was the Soul King or during modern times knew that Gai was housing the genocidal king. Only few knew that Seikyo was within Gai's soul, and even then, none were aware that Seikyo had reanimated the Mototsu-Seishin hybrid. After childbirth, Gai's parents had thought that the doctor had made a mistake with declaring their son as stillborn. Seikyo was awakened once again in a battle with Kazuma some time before the events of Bleach: Rising Phoenix, and later made his true appearance during the final chapters of Part I, where Gai, consumed by grief, tapped into his powers to combat Mitsuru Inazuma.