Edited on 1/17/2019:
I suggest skipping if your OC is a supporting class or villain.
If your OC is a hero:
Unfortunately, you'd gotten wrapped up into this situation. Todoroki, another classmate, had asked you for backup in an alleyway once learning you were nearby. Of course, you had gone to help, and you're now stuck fighting the 'Hero Killer', Stain, alongside Todoroki, Midoriya, and Iida.
(Post your OC's blood type before starting.) The blood type that will be down if Stain manages to get a drop of their blood is type B, then AB, A, and lastly O.