You cannot save everyone.
Those words were written neatly on the back of the cover, right next to his name and contact information in case the notebook went missing.
It was a truth that Kunikida knew he had to accept, hoped that by placing it in front of him this way that maybe he one day could. Instead, he rather saw it as a challenge a sentence waiting to be completed: You cannot save everyone, yet.
Perhaps he never would, but he would try until he could not any longer. Self-sacrifice was nothing if it meant the protection of others.