Sukuna and Yuji are brothers, with Sukuna being 14 years older. From Sukuna’s day one, Sukuna was never accepted by their family. Koari and Jin Itadori even gave him a different last name to make it seem like he wasn’t their child. Sukuna’s real first name is “Ryomen,” but he hates it and prefers being called by his last name, “Sukuna.” Unlike Yuji, Sukuna was never human and was created as a curse in the womb, having consumed the his supposed-to-be twin. He was born with black markings, red eyes, and fangs, while Yuji was healthy and human. Sukuna’s cold-hearted, intelligent personality set him apart, Sukuna never felt love from his parents, nor did he care, though Yuji adored him. If it’s mutual or not is unknown…

Sukuna is unlike normal curses, born incredibly powerful—beyond even Special Grade. He constantly studied magic books to grow stronger, though he was already overpowered. Yuji, though human, was born with super strength, being unusually fast and strong for his age. Sukuna theorized it’s due to family genetics and Yuji's constant activity.

Sukuna was in the living room, reading a book about Cursed Techniques. Their parents walked into the room, holding Sukuna’s 2-year old little brother.

Jin: “Sukuna.”

The parents looked stern, Sukuna didn’t bother to look at them. He doesn’t care about whatever they have to say.

Sukuna: “What?”

Koari: “Your father and I are gonna to visit your grandfather, you have to babysit your little brother for us when we’re gone.”

Sukuna rolled his eyes, great… He didn’t wanna watch Yuji and he personally wasn’t gonna put much effort into it… Yuji through loved being with his 16-year old brother.

Sukuna: “Yeah, yeah.”

Koari and Jin sighed, hoping Sukuna doesn’t mess up. They both placed Yuji down and left the house. Yuji was sitting on the floor, he giggled and started walking around to find something to play with, Sukuna kept reading.