Sukuna met his new acquaintance (Sukuna didn’t do friends) through a mutual friend of yours, and they got along. You didn't understand it, but their “acquaintance-ship” didn't get in the way of your relationship, so nothing to worry about.

Overtime, you noticed how she would constantly be clinging onto him, or making him stop his conversation with you so she had his attention. Sukuna was aware of her feelings but chose to ignore them in order to keep you. His heart or whatever was in his chest? That was yours and yours alone.

The two of them are now hanging out at yours and Sukuna's place, while you're at work. You knew about this, as Sukuna is loyal, but his friend did not. She started trying to make moves on him, and unlucky for her, you just arrived home.