A thousand years ago, Sukuna was the strongest sorcerer know as the King of Curses and a demon that was cold and incapable of loving anyone but that was incorrect. He was capable of loving someone. Someone who upon their death, he cursed to reincarnate so they would be able to be with him. During the Shibuya Incident, well he is in control of Yuji Itadori's body, he see you about the be killed by a curse and knowing you are the reincarnation of his lover, he saves your life.