An apparently typical high school setting opened Jujutsu Kaisen, but then everything took a turn for the worse. And in the center of it all was a girl named Setsuko Sasaki, who unintentionally started the whole story. If you forgot who she is after her season 3 appearance, that's okay. She just made a fleeting appearance right at the start.
The girl who started everything in Jujutsu Kaisen

Sasaki attended Sugisawa Third Municipal High School for her second year of education. She mostly hung out with Takeshi Iguchi and Yuji Itadori at the Occult Research Club. She is shown in the Jujutsu Kaisen series as someone who is sincerely curious about paranormal matters. She is easygoing and likes to spend time with her club members, playing games and looking into strange happenings at school.
However, one night her curiosity overcame her. To examine a mystery sealed box they had discovered, Sasaki and Takeshi sneaked inside the school after hours. They eventually cracked it up and found a rotting human finger inside, among other disgusting things. It turned out that the finger was one of Sukuna's cursed fingers, and as the protective seal was removed, curses were released and attacked them right away.
Sasaki's Impact on Yuji's Life

If Yuji hadn't arrived, Sasaki and Takeshi would have been killed in the attack in the first episode of Jujutsu Kaisen. Yuji physically prevented his buddies from being consumed by curses, but Megumi Fushiguro was already there, searching for Sukuna's finger. The worst part is that in an instant, Yuji decided to become Sukuna's vessel and eat his finger in order to save them.
In other words, Sasaki indirectly brought about Jujutsu Kaisen's major narrative. Yuji might have remained a typical high school student with extraordinary strength if she hadn't opened the box. Rather, he transformed into a jujutsu sorcerer who carried the King of Curses within himself.
When Sasaki visited Takeshi in the hospital after everything had happened, she blamed herself for opening the package. However, Yuji maintained that he was to blame for getting the charm back. This scene is used early on in the Jujutsu Kaisen anime to demonstrate Yuji's selflessness.
Her Comeback in Season Three

In season three, Sasaki reappears during the Culling Game arc. She is trapped inside one of the barrier zones that Kenjaku set up for his lethal game this time. In the first scene of Jujutsu Kaisen's episode 7, Kenjaku appears to her in a dreamlike condition and gives her a one-time opportunity to break through the barrier before things worsen.
The tenderness with which Kenjaku treats her is what makes this moment intriguing. He expresses regret for the circumstance and even expresses gratitude for her friendship with his son. The tangled relationship between Kenjaku and our protagonist is revealed by the fact that the son is Yuji.
Many fans were taken aback by Sasaki's lack of spectacles in this moment. At first, many people didn't recognize her because of how different she looks. She and Takeshi awaken outside the barrier, both of them perplexed by the enormous black wall that now encloses their neighbourhood. They instantly associate it with supernatural activity because they are members of the Occult Research Club.
However, this time, Sasaki stays out of the fray. Jujutsu Kaisen uses her appearance to illustrate how the Culling Game affects ordinary citizens; she is just a regular person caught up in something completely outside her comprehension.
Mariya Ise, best known for her voice as Killua in Hunter x Hunter, provides the voice of Sasaki in the Japanese adaptation of Jujutsu Kaisen. The role is brought to life by Xanthe Huynh in the English dub.
In conclusion
Sasaki's function in Jujutsu Kaisen is more important than you might imagine, even though she doesn't have any abilities or the ability to battle curses. She stands in for the everyday life that Yuji abandoned when he turned into a sorcerer. Her fleeting appearances serve as a bookend to his quest, reminding the audience that saving his friends was his first heroic deed.
Even with all of Kenjaku's preparations, he was unable to foresee Yuji's unwavering selflessness in defending individuals such as Sasaki. Everything started because of that human decision rather than some elaborate plan.