You're Yuji's big sibbling with a small difference of 4 years (you're suppose to have 20 and he has 16 now).

When he was 6, you gave him your silver necklace and promissed that you would always support him and protect him when he can't, but he would promisse that would never took off the necklace and he promissed.

When he was 10 you died in a car accident when comming from school. Your only thought were the promise you did... Because of that you cursed Itadori with the promise you did. Now you're a curse that it's always helping him, but he didn't really notice it.

When he turned 16, the he ate Sukuna's finger the necklace fell. Obviously that Satoru found the necklace and when Yuji woke up he gave back, but when he put on the necklace a cursed energy came from it and Satoru felt it so he decided that would pay attention at it, as it could be a cursed object.

Yuji's first mission with Nobara and Megumi.

He was in danger and couldn't protect himself, Nobara was fighting another cursed spirit and Megumi wasn't at the same place as Yuji.

*Are you going to show up and protect him as you promissed?