Spin, Spin!

Kodai lost his younger sister in "Noah's Flood", a catastrophic event that submerged Tokyo not too long ago. He confines himself to what's left of his old life, refusing to move on. One day, a mysterious girl who calls herself "Kururi" jumps out from the endless sea, claiming that Kodai's "home" actually belongs to her. After a brief chase from some mafia-type group, Kururi forces herself into Kodai's life as his new roommate. Because of this, Kodai is forced to start moving on from his tragic loss and both of them have to learn to adjust to the new way of life that "Noah's Day" forced everybody to live.

The Doujinshi & Manga Lexicon
Vol. 1
Vol. 2
Vol. 3
Vol. 4