18+ Chikyuu no Houkago

Most of humanity has been wiped out by mysterious dark monsters known only as "Phantoms." Four survivors, who may well be the last of the human race, have found one another and are sharing a quiet, peaceful existence. These survivors are a kind, dependable teenage boy called Masashi, two teenage girls, shyer Sanae and bolder Yaeko, and a little girl called Anna. They feel as though they're just living in a perpetual "after school" state, waiting for the rest of the human race to come home. As they pass their days together, they begin to see some strange things and find that the monsters have not yet left the Earth. Will they discover what really happened to their planet?

(Source: MangaHelpers)