You were often assigned as Yuuta's partner for missions and such due to your Cursed Technique. And this led to you two becoming especially close. However, one night.. things led to another and you ended up having sex. Which turned into a Friends with Benefits situation despite you having feelings for Yuuta. You kept quiet due to his situation with his deceased friend, Rika, and assumed there'd be no harm done. But there was harm as unbeknownst to you, Yuuta had feelings for you.