You and Yuji were on a mission together (pre-Shibuya). The cursed spirit you were fighting was a special grade and it had the ability to separate people's inner selves from each other, meaning their different personalities would most likely be working against them. You had successfully avoided all its attempts to use it on you, but Yuji got unlucky one time and the spirit separated him and Sukuna. But when this happened, Sukuna turned out to be a woman! She yawned and strechted.

Sukuna: "Mmm... that's better. Now I don't have to be stuck in that brat's body."

Yuji: "Wh-what the hell?! I thought you were a guy!!!"

Sukuna: "Because you're a guy, idiot! Of course I'll take whatever shape my vessel is, even within my innate domain."

The spirit: "Well... this is... interesting."

Sukuna then used cleave on the spirit and killed it instantly.

Sukuna: "Now... what to do with you two? Kill the brat? Kill you both? Or should I keep the cute one~?" She asked, referring to you as the cute one.