No 6 Vol 5

No 6 Vol 5 by Atsuko Asano. A Sci-Fi manga rated Teen, published in English by Kodansha in 2015.

In the seemingly perfect city of No. 6, gifted student Shion shelters a wounded runaway named Nezumi, a choice that costs him his privileged life. Years later, when a deadly mystery strikes the city, Nezumi returns, and the two uncover the truth about the place they call paradise.