Mizuki Rashojin/Story

Past
1000 years before the series beginning, she was an infant who was thrown into the sea due to a country famine. Since she was abandoned with hatred and was filled with an intense fear before her death, her spirit inadvertently made a pact with the evil entity Ambrosia. She miraculously survived this event and washed ashore the Shimokita Peninsula to a new family. The villagers there named her Mizuki (which means "seeing") since she seemed to possess mystical abilities.

Thirteen years later, the village made a sacrifice and it formally contracts Ambrosia. Mizuki then became an evil entity, Rashojin Mizuki. Since then, she has terrorized the land with various disasters around the area. She evolved to a state where she could host another body and keep herself intact using its energy. 300 years later, she reaches her mental limit and hibernates near a shrine to recover her strength. A century later, she found a shrine priestess, Bizuki, to be a suitable host. From then on, Mizuki fed off Bizuki's heightened spiritual senses. She was sealed into a mountain hundreds of years later by several puppets and monks. Though she appeared to be dormant, she was actually sleeping and gathering her energies.

Thirty years before the events of Samurai Shodown II, she has a brief encounter with Nicotine Caffeine and Kuroko. She was awakened by Amakusa's powers and soon invoked Ambrosia's mortal resurrection. To aid her work, she also hired a band of henchmen which includes Earthquake, Genan, and Genjuro (though he eventually betrays her due to lack of interest). Haohmaru, whom had successfully survived the trip to Hell, stop her plans from succeeding. Upon her defeat, Mizuki and Ambrosia were sealed in the Makai (demon world).

Shinsetsu Samurai Spirits: Bushidou Retsuden
In the RPG, Mizuki was a rural Japanese child abandoned by her family due to severe poverty. Bitter with hatred and consumed with revenge, she made a pact with the evil god Ambrosia roughly a thousand years before the series begins, turning herself into a demon. Since this acquisition of demonic power, she proceeds to cause worldwide disasters and calamities around the globe. However, roughly 800 years after her pact, she was challenged by a priestess named Bizuki. Despite her best efforts to purify the entity, Bizuki was possessed by Mizuki and sent into hibernation. When the party faces and defeats Mizuki, her body disintegrates into a skeletal form. She summons Ambrosia's grotesque body to finish off her adversaries, and is defeated.

The King of Fighters XIV
Verse's defeat during the events of The King of Fighters XIV had released numerous souls and has caused many who have perished throughout the series to be brought back to life. Mizuki is one of the souls who came back to life.

The King of Fighters XV
Due to Mizuki's revival, Nakoruru travels back in time to ask Haohmaru and Darli to slay her during the fifteenth tournament. In the Team Samurai ending, Mizuki was about to defeated once more by that team, but they decided to change their minds much to her confusion and frustration, deciding to tag her along in exploring the modern world. In their post-credit scene, it’s seen that Mizuki has quickly adapted to the modern ways of the world alongside Haohmaru and Darli, thanks to Nakoruru's earlier exposure to it.