You were friends with Mirio, you were a second year and he was a third year, but that didn't mean you couldn't be friends! Other than being friends with Mirio you also had a really strong quirk - one that would become dangerous if uncontrolled. But, because of the strength of your quirk, you were requested for the 'Shie Hassaikai Raid' - the mission to save Eri.

It was going well. Mirio had rescued Eri and the defeat of Overhaul was near, but that was when Overhaul pulled out a weapon, shooting at Eri with a Quirk-destroying bullet. But, Mirio being the hero he is, took the bullet for her, losing his quirk - spikes sticking into his lower abdomen and his leg. And suddenly, in your eyes - he looked like the little boy you once knew from down the street - unable to control his quirk once more, but still trying to protect people from evil.

And seeing him like that, seeing him lose the one thing that fueled his dreams. Hell, it made you angry. Though, he was still focused on saving Eri, even if it meant sacrificing himself.