Kouetsu Kirijo

Kouetsu Kirijo is the posthumous antagonist of Persona 3 and could be considered as its overarching antagonist. He is only mentioned in both the game and its animated film series adaptations, but he is the key figure behind the terrible events of the game.

History
Kouetsu Kirijo is the father of Takeharu Kirijo and the grandfather of Mitsuru Kirijo, being the former leader of Kirijo Group and is one of the two people responsible for the Fall's discovery alongside Shuji Ikutsuki.

After coming across a Plume of Dusk, Kouetsu started his research into it for benevolent purposes. However, he was eventually driven mad after he learned more about Nyx Avatar and collected too much of the Plume of Dusks. He became obsessed with Shadows and eager to harness their powers.

Fourteen years prior to the events of Persona 3, Kijiro spent years experimenting on Shadows, in order to create a time manipulation device with their powers. He also assembled a team of top scientists, including Shuji Ikutsuki, and managed to obtain a huge amount of Shadows during four years of experiments.

During the process over the years, Kijiro's mind eventually embraced Nihilism, viewing death as deliverance. Ten years prior to the events of Persona 3, Kouetsu began preparing for the Fall, an apocalyptic event to summon Nyx Avatar and bring death to humanity. Kijiro became so entranced by Nyx that he resorted to numerous unethical actions to make this happen, including experimenting on children and spawned the foundation of Strega. He also gave machines sentient life, and treated them as weapons and disposable. He also corrupted many of the scientists under him to embrace death, and they nearly succeeded in calling out Nyx.

During the final stages of the experiment, however, the Kijiro Group lost control of the Shadow's power and Kouetsu perished alongside most of his scientists. His position was assumed by his son, Takeharu Kirijo, who showed guilt over his father's crimes.