Classmates stuck in an apocolyptic city. Strong one had a crush on the weak one for a while, but weak one has a girlfriend (who beacame a zombie and s knows but not w) and he is searching for her. He gets bit by his gf who dies by s, and s chops w's arm off to stop the infection. Now s lives with the traumatized w.
Its so good
Uruwashiki Shuen