It's not quite as straightforward as that, however -- throughout the series, it's ambiguous exactly what the nature is of the characters' relationships, realities, or, in the case of
Griffin, sanity.
In the first book, Griffin appears to have imagined up Sabine in a fit of utter loneliness and depression, only to have his other, "Sabine" personality take dangerous form in his mind. Later, the question of her reality and, if real, phasing into Griffin's plane of reality, is still vague.