THERE MAY BE SPOILERS - WORSE THERE MAY BE THINGS SAID WHICH SPOIL THE STORY - I WOULD SUGGEST READING IT FIRST AT
She Gave Her Heart, He Took Her Marrow by Sam Fleming.
Chancery is autistic (or has Aspergers) - one day her only friend in the world, Annabel, decided to leave. Desperate for her to stay, Chancery went outside, experienced the strange events that followed, and which ended up with Annabel going to the shore and drowning herself. And there she meets Hedron, who saves her from pain, and... Hedron is clearly an odd person himself, who has no idea why, for example, the people who go into the sea are drowning. He's aware that Chancery is different because the others don't try to talk to him either. He keeps her alive. For five years with no one else around, but the Oilers, who come for the oil and who Chancery trades with for things like chocolate. Theirs is a strange but symbiotic(?) relationship. Eventually Chancery meets Kay, an Oiler who isn't people but a potential friend, and welcomes her despite Hedron's annoyance. Kay may be right that Chancery is fading, but she wrongly believes that Chancery survived because she is immune to the spores which cause others to Walk (there are some zombie-like remnants of people who are Hedrons eyes and ears and guards). When Chancery refuses to go back with Kay, she is kidnapped, and faces a future of medical testing, except Hedron comes to fetch her. Death follows for everyone but Chancery, and Hedron makes his intent to keep her alive and happy toothily clear. If she is, indeed, failing as Kay thought, I'm definitely not hopeful about where that goes.
It's an excellent idea, with a little more expanded weird than I was entirely happy with and that Chancery wasn't going to explain. I wonder if the first part, followed by the flashback and trail forward was better than starting at the start, but then Kay would have arrived halfway through... on the other hand it's difficult to feel concern for a character's previous life-threatening danger when you already know they survive. It might also have helped bridge the slight distance from Chancery which bugged me a little.
It's an idea driven story with a little weird romance that felt like it could have been a little tighter wound and wrapped.
And I don't think I'd have put it at the end of the book.
But then I'd have swapped out at least five stories so...