Warning: The following aticle contains spopilers from the Jujutsu Kaisen Manga. Read ahead at your own discretion!
A character that intrigues most people in Jujutsu Kaisen is Geto Suguru. He is a mysterious person who was a shaman earlier but turned into the leader of curses.
Aside from hating humans for being weak, he also has meticulous plans of using the curses at his disposal to the dirty work for him.
Geto Suguru has a vision of eliminating all humans to realize the age of jujutsu sorcerers.
However, the recent chapters showcase a wholly different path of this “Geto Suguru.” This person reveals that Geto Suguru’s body is possessed by a mysterious brain.
This means that not all of his plans went as expected, and he died much earlier. When did this happen, and how?
What happened to the real Geto Suguru?
In Jujutsu Kaisen 0, Gojo Satoru kills Geto Suguru after he attacked the Tokyo Jujutsu Tech school. However, Gojo asks Shoko Iori to not dispose off Geto’s body, which enabled the suspicious brain to take over.
So what went down to make things so bleak?
To bring his goal of killing all humans except jujutsu sorcerers, Geto Suguru wanted a special grade cursed spirit on his side. This cursed spirit was Rika Orimoto.
To get her, he decided to kill Okkotsu Yuuta. By killing the master, he could ingest Rika and control her as desired.
After his bold declaration of war, he successfully isolated Okkotsu. Even though he faced a few “minor setbacks” in the form of Panda, Inumaki, and Maki fighting him, he quickly defeated them.
The relentless attacks rendered them unable to resist. Seeing the plight of his friends, Okkotsu called Rika forth in her true form to fight Geto.

Geto Suguru was extremely sure that he could kill Okkotsu. However, he had gravely underestimated Okkotsu’s determination and growth over his time in the school.
He had learned to fight using Rika and her cursed energy.
Geto escapes this fight barely clinging on to his life. However, Gojo ends up finding him in an alleyway. Despite his visible hesitation and the past shared with Geto Suguru, Gojo kills him.
If Shoko had disposed of Geto’s body as per procedure, then the brain probably couldn’t have taken control of Geto. However, it seems that something (probably his relationship with Geto) made Gojo think otherwise.
And so, here we are!!

But then why did Gojo come so late, where was he all the while? What was the “war” declared by Geto?
What is the Parade of a Hundred Demons In Jujutsu Kaisen?
He declares that at sundown on December 24th, he will carry out the Parade of a Hundred Demons. A thousand cursed spirits will be released at two locations: Shinjuku and Kyoto.
He will issue them orders to massacre anything and everything in their way.
However, this declaration leaves Gojo, and the others confused. He might have over 2000 curses with his forces, but all of them, grade 2 or lower. This would mean a war that Geto couldn’t win at all.
Geto’s master plan was to distract Gojo and the other jujutsu officials while carrying out his real strategy of isolating Okkotsu. For this, he made use of the shamans’ biggest weakness: their duty to protect humans.

Of course, all of it was a bluff. Geto knew that the probability of winning this war was 30% at most. Only one possibility could shoot up the chances to 99%: killing Okkotsu and ingesting Rika.
So, while the others were preparing for a war with the curses, Geto prepared for a whole different battle.

The only fight at the actual location was a few Geto’s followers buying time. Once Ijichi and Gojo realize that something entirely different is going on, Gojo sends Panda and Inumaki to help Yuuta.
This “help” was actually a challenge to Okkotsu in disguise. Gojo knew Panda and Inumaki could not defeat Geto but would draw out Okkotsu’s potential.
Gojo takes care of the situation at hand before rushing to kill Geto.
TL;DR
At the end of the prequel arc, Geto is killed by Gojo. His body is not disposed off for some unknown reason, which leads to a Brain taking over.
The real Geto died long ago before the current events in Jujutsu Kaisen. Brain, or the impostor, only used Geto’s abilities and knowledge to realize his own ambitions of an ideal society.
What did you think about Geto’s plan? Was it close to a master stroke? Let me know.
great analysis.
i wonder why gojo had to kill him in the end. was it because he wanted to stop him from degenerating more?
i’d love an in depth analysis if the real reason gojo decided to kill his only dearest true best friend.