I know it's just Yotsuba... Besides, that's not like I like particulary this mango but now, I'm starting to imagine Koiwai as a soldier, fighting desperately for his life.
After a MANLY BATTLE where all his unit is killed, he found Yotsuba, crying and shaking because of all the madness she just saw. (In before, MADNESS?)
Thanks to her, he cries MANLY TEARS, because he finally understood how war is meaningless. When he's about to kill himself because he lost his reason to live, Yotsuba is grinning to him, to cheer him up.
In the last panel, we see Koiwai who leave the battlefield, a rifle in the right hand to protect Yotsuba who is holding his left hand. His final words are "This war wasn't so meaningless after all"