I rejected my fate to be born as twins. My foolish mother must've been starving. So before I also starved, I ate my other half inside her womb. My other half's soul must've been wandering until they were used by Kenjaku. You should know this, but Itadori Yuuji as a culling game player was born with a cursed object (my finger) sealed within him. It must've have been necessary to ensure his strenght as a vessel.