This is a sci-fi book I read in high school. A family or group of space explorers crashed on an alien planet before the book begins. Aliens manage to save the girl by altering her body to survive on the planet, but all the other humans die. The aliens raise the girl as best they can.

Later, another group of humans arrive to explore the planet, and they're shocked to find the girl present. The girl has to wear an environment suit because where the humans are the girl cannot survive because the atmosphere is too thick. She develops a crush on one of the boys in the group of humans who arrive. Eventually, she reveals herself to the other humans, and finds that the aliens horribly disfigured her in the attempt to save her.

Yes, it's sort of like the old Star Trek pilot, The Cage, except this is told entirely from the girl's perspective.