The story is set in "No. 6," a future model city in 2013. Raised as a top-ranking elite since the age of 2, a boy named Shion lives in an exclusive area in No. 6. On the night of his 12th birthday, he rescues a boy who calls himself "Nezumi" ("Rat"). Nezumi ran away from the city's Special Security Area. From that moment on, Shion's life is changed forever...
Another one of those I read years ago and I LOVED IT! It's still extremely high in my favorite series EVER. Shion and Nezumi are so sweet to each other and the fact that we didn't get an exact happy ending literally broke my heart.
No. 6