Classmate, Kamimura Yuuka wa Kou Itta.

Shuuichi is a normal guy who decides to listen to the weird girl of the class, Kamimura Yuuka, who ends up revealing "the truth of this world" to him. This is where his everyday life gets shaken up and he starts to help her uncover how much of what she believes is true and how much is what it seems to be. On their way to find who is behind everything they fight/flee from giant robots, ask out anyone they can, find allies and try to understand just why they are the way they are.
While doing so, the manga tackles themes like the reality of one's feelings, what it means to be a human, death, regrets and love.
We read about how Shuuichi feels about what he'll learn about the world, how he feels about Yuuka's role in all of it, how the people around him feel about what they find out and most importantly: how he feels about himself and who he truly is.
We read how Yuuka sees him, considering what she knows and what she'll find out, how she felt before meeting him, how she feels in a world where she doesn't know how much of what she does is her own choice and how much was decided by others, how she feels about the reality she'll have to accept and how she feels about Shuuichi after the revelation of the truth.
We learn about how much each character is willing to sacrifice in order to reach their goal and why they'd rather go after the things that might kill them, the people they can't fight, why they'll give up their everyday life in order to find a truth which's fragments they already have troubles accepting.
We'll see the imperfection of mankind and it's attempt to be perfect, we'll see how behaving like gods will lead people away from their original goals while watching the conversations (times funny, times serious) of an overall lovable cast with Yuuka as it's leader. We will see how important each character is for the other, how they teach each other, how well they understand the people around them and how they handle their conflicting goals and wishes without ruining their friendship.
While I think that those things were beautifully done, I should warn you from things like mediocre humor, Shuuichi being pretty boring he's not horrible and I liked the end a lot, he's just nothing spectacular and I don't think he should be as convinced at the end as he was, that's just me tho and it was in-character for him to believe what said),

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the fact that we'll never know what exactly was done to the former test subjects, I'm not asking for visuals but showing a few short moments of their time in the city would be nice

and the fact that each chapter has something like 40 pages instead of the traditional 20, otherwise it's a great manga that I enjoyed a lot.