The Real Reason Behind Yuji's Connection To Sukuna In Jujutsu Kaisen
Yuji Itadori, the protagonist of fantasy anime "Jujutsu Kaisen," begins its pilot episode as a human high schooler with little connection to the world of the supernatural other than a personal interest in the occult. By the end of the anime's first season, Yuji is a powerful fighter, capable of single-handedly dispatching monstrous manifestations of negative human emotions — called "Cursed Spirits," or "Curses" — that threaten humanity from society's metaphysical fringes.
At the core of Yuji's development from standard-issue human to magic-wielding soldier is the fact that his body houses the spirit of Ryomen Sukuna, the most notorious Curse in the history of Cursed Spirits. Yuji becomes a vessel for Sukuna in Episode 1 when he chooses to swallow one of Sukuna's fingers — the only vestiges of his earthly body that remain, of which 20 exist in total. He does this in order to attune himself to the supernatural world and fight back against a Curse that threatens his school and new acquaintance, Megumi Fushiguro.
Soon, Satoru Gojo, the most powerful among humans dedicated to fighting Cursed Spirits, appears on the scene to assess whether or not Yuji should be allowed to live, given that Sukuna's revival is now predicated on his existence. Gojo then identifies one key characteristic that justifies Yuji's survival, which becomes responsible for his sustained connection to the spirit of Sukuna that his body houses.
Yuji's willpower is responsible for his continued relationship with Sukuna
When Sukuna first manifests in Yuji's body in Episode 1 of "Jujutsu Kaisen," Fushiguro fears that this means the reappearance of Sukuna in the world, and thus a crisis of the highest order. Yuji, however, regains control of his body shortly after, acting as if his ability to suppress the influence of one of the most powerful dark spirits of all-time through willpower alone is unexceptional. Then, when Gojo arrives on the scene, he tests Yuji's ability to manage how long Sukuna can remain in control of his body. Yuji passes this test, regaining control from Sukuna at a predetermined point.
Yuji's powers gain him a protected status in the world of Jujutsu Sorcery after Gojo convinces those in charge that Yuji can serve as a vessel for Sukuna's remaining fingers, given that their influence is seemingly incapable of overtaking him. Their agreement likewise involves killing Yuji once he consumes all 20 of Sukuna's fingers, but this isn't yet close to becoming a concern as of the end of the first and thus far only season of "Jujutsu Kaisen." Rather, for now, Yuji's preternatural ability to suppress the influence of a Cursed Spirit within him means that he and Sukuna are intimately linked until Yuji gathers and consumes each of his fingers.