Here’s the information about the evil curse user Suguru Geto (夏油 傑 げとう すぐる); the relationship with Satoru Gojo, why he has the scar on his forehead, Jujutsu technique, quotes in Japanese and more!

Name: Suguru Geto (夏油 傑 げとう すぐる)
Age at death: 26
Birthday: February 3rd
Grade: Special grade cursed user
Likes to do: Martial arts
More info:
・He was scouted for Jujutsu tech.
・Favorite food: Zarusoba
・Food he doesn’t like: None
・Stress: Taking cursed spirits in body

To write about Geto, I separated my writing for “Before Hyakkiyako (百鬼夜行 ひゃっきやこう)” and “After Hyakkiyako” because he was killed by Gojo in the Hyakkiyako incident. So, it means Geto before Hyakkiyako and him after Hyakkiyako are the different person.
Before Hyakkiyako

1. The relationship with Satoru Gojo
They used to be classmates.
Geto, Gojo and Shoko Ieiri (家入 硝子 いえいり しょうこ) were classmates in Tokyo Jujutsu Tech (Their teacher was Yaga.) You can read the story back then in the comic vol. 8 and 9. Even though Geto and Gojo sometimes got into arguments over their sense of values, they admitted each other’s strength and high Jujutsu technique. It seems they had a good relationship as rivals.
2. Gojo considers him as a best friend.
In the comic 0, Gojo said about Geto “my only best friend”. Gojo even changed the way he called himself influenced by Geto.
For more details, see my articles → Why Gojo Change The Way He calls Himself: Analysis Of The Relationship With Geto 【Jujutsu Kaisen】
3. Trigger to become a curse user
Geto’s thought totally changed after the mission of escorting the star plasma vessel. The mission failed and he had to face the death of the innocent girl because of the selfish people’s ego. Since then, he started doubting the role of Jujutsu sorcerers and having a feeling of disgust about non-sorcerers.
While Gojo was blooming his Jujutsu talent, Geto was losing himself and the purpose of being a Jujutsu sorcerer. During the time, he had a chance to talk to the special grade Jujutsu sorcerer Yuki Tsukumo (九十九 由基 つくも ゆき) and got an idea of sweeping all non-sorcerers out to make the world without cursed spirits.

Finally, Geto killed all 112 people in the village where he was on a mission and disappeared. Since then, he was identified as a curse user to be executed according to the Jujutsu regulation article 9.
4. Hyakkiyako (百鬼夜行 ひゃっきやこう) Dec. 24th, 2017
Geto appears in the comic 0, the prequel of Jujutsu Kaisen currently serialized On the Shonen Jump Magazine. In the story, Yuta Okkotsu (乙骨 憂太 おっこつ ゆうた) is the main character instead of Yuji Itadori (虎杖 悠仁 いたどり ゆうじ, Yuji doesn’t come out in the story, yet.) and Geto was already the curse user at this point, So Gege sensei had the concept of Geto from the very beginning.
In the comic 0, Geto caused a big incident called Hyakkiyako. He said he was going to release each 1000 cursed spirits in Shinjuku, Tokyo and Kyoto to attack non-sorcerers. But his true purpose was to take over the special grade cursed spirit Rika who possessed Yuta. However, Geto was severely injured by Yuta and Rika, and soon after that, he was killed by Gojo.
After Hyakkiyako
5. Fake Geto
Suguru Geto was dead in Hyakkiyako in 2017. But he appeared in the main story which started from 2018. That’s because the one in the main story is not Geto. Actually, the body is Geto’s own body, but it was taken over by the curse user Kenjaku (羂索 けんじゃく).
Kenjaku can take over one’s body by changing the brain to his and this is why Geto in the main story has the scar on his forehead. He’ had been alive for at least 1,000 years by changing bodies.
Here's more derails about Kenjaku ↓
6. Joining hands with the cursed spirits
Fake Geto has joined hands with the special grade cursed spirits, Jogo, Hanami, Dagon, and Mahito to make the world where cursed spirits dominate instead of humans though Geto’s purpose actually lied somewhere else.
7.Shibuya Incident (渋谷事変 しぶやじへん) Oct. 31st, 2018

Shibuya Incident is an unprecedented incident caused by Geto and his followings to seal the strongest Jujutsu sorcerer Satoru Gojo into the special grade cursed tool, Prison Realm (獄門疆 ごくもんきょう). As the result, 23 wards of Tokyo were almost wiped out and Geto’s side got a big damage, too. However, they succeeded in sealing Gojo and they literally brought chaos to both the society and the Jujutsu world.
8. Culling Game (死滅回遊 しめつかいゆう Shimetsu Kaiyu)
Shimetsu kaiyu is the evil game Geto (Kenjaku) planned to involve non-sorcerers in. His ultimate goal is to have Master Tengen (天元 てんげん) fuse with all human beings in Japan. Tengen is the Jujutsu sorcerer with Jujutsu technique of immortality. His barrier protects Jujutsu tech and all barriers for each Jujutsu tech, in Jujutsu world and by assistant Jujutsu sorcerers are strengthened by his power.
Personality
In contrast to Gojo’s carefree personality, Geto is a strict person. When he was in the 2nd year of Jujutsu Tech, he already believed that Jujutsu sorcerers existed for non-sorcerers because strong one should protect weak ones. So, we can tell that he had pride and responsibility in being Jujutsu sorcerer from early on.
According to the official fan book, the author Gege Akutami sensei said he thought of Geto because he wanted to draw a character who had a biased thought.
In the chapter 4, Satoru took his students, Yuji, Megumi and Nobara to their first mission (Megumi didn’t join the mission.) and he explained to Megumi that Jujutsu sorcerers must be crazy enough to stand the fear and hate during their hard work. I guess Geto was a person who had a good common sense too much to be a Jujutsu sorcerer. He wasn’t crazy enough to go over the unfairness of circle of life.
After he left Jujutsu Tech, he called non-sorcerers, “monkeys” and hated them so much that he was using a deodorizing and sanitizing spray to himself after he talked to non-sorcerers.
He even killed his family members by own because they were “monkeys”, too.
On the other hand, he was basically nice to ones with cursed energy. In the comic 0, he called the curse users who followed his thought, “my family” and also didn’t kill Toge Inumaki and Panda in Jujutsu Tech even though he could have. I think he was not just evil but too extreme following his ideals.
*Note: this is about Geto’s personality. Since his body was taken over by Kenjaku after Hyakkiyako, he is not Geto anymore.
Jujutsu Technique

Cursed Spirit Manipulation (呪霊操術 じゅれいそうじゅつ)
Geto can compress a cursed spirit into a small ball and stock it inside his body by swallow it. He can take it out whenever he wants and handle it. How many cursed spirits he can stock at once is unknown, but he said he had 4461 cursed spirits in his body in the comic 0.
Goku no ban 「Uzumaki」 (極ノ番 「うずまき」)
Geto can combine the cursed spirits he has into one. By using Uzumaki, he loses the spirits he has, but it has a big advantage which covers the disadvantage: the extraction of Jujutsu Technique. At the battle of Shibuya incident, Geto used Muitenpen (無為転変 むいてんぺん), the technique of Mahito (真人 まひと) who he had taken in right before.
*In the comic vol.16, the author Gege Akutami sensei noted that he got an idea of Uzumaki from the masterpiece of horror manga “Uzumaki” by Junji Ito sensei.
Quotes

祓 (はら)う 取 (と) り込 (こ) む
Source: Gege Akutami 『Jujutsu Kaisen』Vol.9
誰 (だれ) のために?
Exsocise, Take in, For who?
Harau Torikomu Dare no tame ni?
To himself
Geto’s mind was breaking down after the mission of escorting the Star Plasma.
術師 (じゅつし) というマラソンゲーム
Source: Gege Akutami 『Jujutsu Kaisen』Vol.9
その果 (は) てにあるのが仲間 (なかま) の屍 (しかばね) の山 (やま) だとしたら?
The marathon game named “Jujutsu sorcerer”. What if its end reaches to the piled dead bodies of friends (Jujutsu sorcerers)?
Jutsushi to iu marason gemu Sono hate ni aruno ga nakama no shikabane no yama dato shitara?
To himself
Yu Hibara (灰原 雄 はいばら ゆう) was a student in Jujutsu Tech and died during a mission. His death even brought Geto close to his breaking point.
Here, Geto chose the kanji “仲間” which meant “friend” but he read it “Jutsushi (Jujutsu sorcerer) instead.
殺 (ころ) したければ殺 (ころ) せ
Source: Gege Akutami 『Jujutsu Kaisen』Vol.9
それには意味 (いみ) がある
Kill me if you want. There should be a meaning.
Koroshitakereba korose Sore niwa imi ga aru
To Satoru Gojo
Geto and Gojo once met in Shinjuku after Geto left Jujutsu Tech. This is the line Geto said to Gojo when Gojo was about to attack him. We can tell what Geto was doing was too extreme but he was not a self-centered person.
私 (わたし) に従 (したが) え
Source: Gege Akutami 『Jujutsu Kaisen』Vol.9
猿共 (さるども)
Obey me, monkeys.
Watashi ni shitagae saru domo
To the followers of the star religious group
The moment Geto vented his hatred to non-sorcerers he called “monkeys”.
ただ この世界 (せかい) では 私 (わたし) は 心 (こころ) の底 (そこ) から 笑 (わら) えなかった
Source: Gege Akutami 『Jujutsu Kaisen 0』
Just, I couldn’t laugh from the bottom of my heart in this world.
Tada kono sekai dewa watashi wa kokoro no soko kara warae nakatta
To Satoru Gojo
This is what Geto said right before he died. This line reminds me of what Megumi Fushiguro said” Karma is not automatic. A bad guy would be judged only after he is brought to the court. Jujutsu sorcerers are one of the cogs of the “Karma”. If Geto wasn’t the cog which had to bring Karma to people, his life would be totally different and maybe he could have laughed from the bottom of his heart there.