Earthlings by Sayaka Murata
Natsuki isn't like the other girls. Together with her cousin Yuu, she spends her summers in the wild Nagano mountains, hoping a spaceship will take her home. When a terrible sequence of events threatens to part the cousins for ever, they make a promise: survive, no matter what. Now, Natsuki is grown. She lives quietly in an asexual marriage, pretending to be normal, and hiding the horrors of her childhood from her family and friends. But dark shadows from Natsuki's past are pursuing her. Fleeing the suburbs for the mountains of Nagano, Natsuki prepares herself for a reunion with Yuu. Will he still remember their promise? And will he help her keep it? A dark and magical reckoning with what it might take to survive a shattered life, Earthlings is an exhilarating cosmic flight that will leave you reeling.