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 the approval of Ikutsuki's Persona experiments on children, which eventually spawned the foundation of Strega. He also gave machines sentient life, and treated them as weapons and disposable, an act that spawned the birth of Aigis.
During the final stages of the experiment, however, the Kijiro Group lost control of the Shadow's power and caused fatal destructions. Kouetsu perished alongside most of his scientists except Ikutsuki, and nearly all of his scientists had became obssessed with pure nihilism except Eiichiro Takeba, the father of Yukari Takeba, who was against the idea of the Fall and made a recording video warning the world about the Death Arcana, a recording that would only be revealed ten years after the tragedy.
After his death, Kouetsy's position was assumed by his son, Takeharu Kirijo, who showed guilt over his father's crimes. His grandaughter, Mitsuru, also showed a heavy heart on the misdeed of Kirijo Group.
Persona: Be Your True Mind
Snow Queen Quest
Persona 3 FES
Persona 4: Arena & Persona 4: Arena Ultimax