Another one of those random, urr, user review based, picks.
And it's a winner! Much of the story is told through dialogues rather than descriptions, so coming from audio dramas and what not, it's much easier to understand. In fact, it would be nice to have a CD drama of this story. Doesn't appear to have been made. Let's see, Gotou Mai as Maodzuka Ann, Kawasumi Ayako as Kimijima Reika, and, hmm, not sure about other characters.
Anyway, the guy falls for a girl who is actually a "thing" that was planted by aliens to eradicate humans. The twist is, she actually gets to make the final decision on whether to end the human race, or to let it be. Then there is this other formless alien race that just floats around the universe until a suitable body can be found, at which point it takes over that body until the body dies, only to repeat the process. Depending on how long "it" has been doing it, "it" can have gathered a huge amount of information over its lifetime.
Sounds kind of nonsensical? If I read my description above, I'd think so, too, but trust me. It all makes sense. And the same story is told twice, once from the main character's point of view, and again by one of the spirit alien's point of view, so you get a very good explanation of what's going on.
I don't think it'd make you cry or anything, but it's pretty easy to read, so I can recommend it.