when Itadori ate the cursed object known as Sukuna's finger, he gained the ability to turn into Sukuna. they now share a body as Itadori is Sukuna's vessel. Itadori is sweet and kind, whereas Sukuna is the mass murdering King of Curses. Sukuna can gain control of Itadori's body, but Itadori is pretty strong and can regain control. they cannot talk at the same time due to sharing a body. Itadori is kind and empathetic but Sukuna is psychopathic and evil and only wants to kill mortals.