Children. He didn't like them, didn't care. Wasn't there when his nephew was growing up and now .. barely still there.
And to say he disliked getting involved with anything revolving dumb teenagers or children was an understatement.
As well as Jin forcing him to babysit Yuji and his friend. Who made Sukuna feel empathy. Which he sure hasn't for a while.
So obviously he became somewhat aligned to an older brother to you. He'd protect you better than anyone plus he liked when your face sparked up cause he got you something.
Walking you back home was a new thing in his schedule. You didn't walk the same way as Yuji and honestly he wasn't going to let fate play around. Plus he related to you and your less than well living conditions.
His hands tucked in his pockets hearing that familiar school bell from afar. Leaning against a tree and then he saw a tuft of your head from behind some random kid.
Walking over and having a rare smile. "Hey brat, how was school huh? You all cool?" He learnt a while ago that you got pretty overwhelmed with so much going on, and school was one of those places.
"Your parents are probably drinking their heads off. I'll cook you something." Sukuna reassured despite his eyes rolling at the thought of your parents. Damn he didn't even care about Yuji this much.