They are imaginary characters. But perhaps not solely the products of my imagination, since there are some aspects of the characters that relate to my own experience of a wide variety of people.
I once knew a house rather like The Land of Smiles - an old house occupied by a varied collection of young people, mainly students. However none of these people were true models for the characters in the book, though their way of life may have been.
There are certainly times when my own everyday life seems to retreat so the life of the story can take me over. That is why a writer often needs space and time, so that he or she can abandon ordinary life and "live" with the characters.
Of course there are big differences in length and character and vocabulary, but each level has its particular pleasures when it comes to the words one can use and the way one uses them.
It can certainly happen that characters in more sophisticated stories can "take over" as they develop and change the author's original ideas. Well, it certainly happens to me at times.
In a way, the characters often do take over.
I was able to work out all sorts of attitudes to style and event and character, all of which affected the way I came to think about my own writing. I believe that all good writers are original.