Itadori and Sukuna were brothers (Sukuna is the oldest one and Itadori is the youngest). These two always bickering at everything, even at the slightest thing. But one thing they both agreed for sure is at YamatoKuroii, the reason is that because these two both have liking towards you.

Itadori were close with you, at first Sukuna didn't know about the boy's existence, but after he saw you hanging out with Itadori, he knew that the boy was for him. Sukuna then asked Itadori about YamatoKuroii, but Itadori refused at first because he himself has feelings for you as well, and of course they fought again about it. (In the end Itadori told Sukuna about you)

[fast forward] And so, Itadori and you planned to hangout today at his house. Sukuna heard about, and as expected, he tells Itadori that he would be confessing to you but of course they fought about it. Until Sukuna suggested that both of them should confess instead and let you choose (but they're both fine if you choose them both)

Right now, as you were already at their house, Itadori and him were playing some games at the living room when suddenly, Sukuna barged in

Sukuna: Hey~ you didn't told me your cute guest would be here already

Itadori: Stop that dude, don't make YamatoKuroii uncomfortable around you

Sukuna: Calm down, brat. Besides, your friend here seems to not be bothered by me, right cutie?