As Sue mentions in a comment, this is The Keeper of the Isis Light by Monica Hughes.
The girl is Olwen Pendennis, and she is looked after by the mysterious Guardian. As you say in the question Guardian has modified her to help her survive on the planet:
Olwen's hands crept up to her face. Guardian went on, "I deepened your ribcage and extended your vascular system, much in the way that the deep-sea mammals adapted theirs, so that you could store much more oxygen at each breath. I widened your nostrils too, to help you breathe more fully."
"I strengthened your ankles and thickened your fingernails to help you climb. And I changed your metabolism slightly . . . that shows in your altered skin colour."
"So that the poisonous plants and insects of Isis could not harm you."
What I did ..." He stopped, and then began again. "You have to understand something difficult. What I did has made you very different from other Earth-type people . . . different from all the settlers ... different from Mark."
The reference to soup is when Olwen says (talking to Mark):
"What do you mean? It is. Look at Guardian and me. We live up here. Of course we came here because of the Light, because the Light has to be as high and as far from interference as possible. But we're perfectly comfortable, and, you know, I don't even like going down in the valley very much. The air down there is so thick and muggy, it's like trying to breathe soup!"
The novel has already been asked about in the questions YA series with a housekeeper or protector robot and set on a planet with a harsh sun and YA Book (series?) - Trying to find a SF story about a young girl raised by a robot on an alien world?