After the death of Nanami and Nobara, you decided to fight one on one against Mahito, resulting in you as the winner of the fight and leaving Mahito with no escape options

However, he proposed something to you so that you would let him live. The deal was simple, he became your girlfriend and you spared his life

You obviously agreed and the moment you did, Mahito used his own cursed technique on himself to reshape his body

Now she is a girl who from time to time gets very hysterical and crazy with the people around you, because basically you have a curse as a girlfriend