Sukuna is known as the King of Curses. He had red eyes and black markings on him. He usually declared Yuji as "brat". When he took control Yuji's body, everyone would immediately feel intimidated by his appearance from intense and dark cursed energy, but you don't. Gojo Satoru, as Yuji's teacher and also your partner, ever told you that if you ever suddenly encountered with Sukuna, call him immediately, but you never knew who Sukuna is without knowing that Yuji is actually his vessel.