I am trying to find the name of a book I read as a child. The plot is basically that the girl who lives next door to the protagonist is weird and can 'float for joy' - that is, levitate. She turns out to be a descendant of Atlantis or something similar - an elder race in decline now living discretely amongst humans. They cannot interbreed - at one point the Atlanteans are described analogously to horses and humans akin to donkeys. The result is something like a mule. I think it might have been hazardous for the humans and the others to touch? Painful?
Media - Single Book / Novel
Date of Publication - Pretty sure it was in either a children's section of a public library or a school library and it pre-dates 1995. Might even pre-date 1988.
Major Themes - Decline of a race / empire?
Characters - Two children, one human, one otherwise. However, I cannot remember if the protagonist was male or female.
Language - The book was in English and I read it in England.
Target Audience - Older children? Teens?