Earthchild

She called herself Reee and she was the last human being on Earth. This was the one thing she was sure of. Because Earth was not a dead planet, not by a long way. There were all manner of strange plants and bizarre animals, and there were the blue boys who insisted they were human - but she always set fire to them. There was however Indigo, the all-devouring protoplasmic ocean that was literally gobbling up everything in the world.
And there was the enigmatic Emeroo to whom she owed her continued existence. There were also the so-called Martians - humans who had fled to Mars and only came back to Earth to scout for survivors and vent their futile furies on the inhospitable homeworld.