This manga instantly reminded me of Deadman Wonderland. Mostly because of the art, but also because of the setting. I mean, it starts out with his friends and classmates getting killed right in front of him. Then the female protagonist turns into something/someone that looks a lot like Shiro. But after looking into it, it turns out it is being created by Katou Yuuichi who worked on Eureka Seven AO (The protagonist from Hiniiru looks a lot like Ao).

Anyway, here's the premise;
Ten years ago, a black shape appeared in the sky and destroyed the city below. Kagerou and Momiji were children when the destruction took place, and they have grown together into teenagers beneath the mysterious structure. There is an undeniable attraction between these two friends, but Kagerou is hesitant to confess his feelings because he feels that he is too weak to protect her. One day, the structure acts again, sending down strange balls that eat people and transform them into monsters. Kagerou and Momiji's encounter that day changes them forever.
And some pictures;

Here's one from Deadman for context as to why I think they look kinda similar.




And from Hiniiru.