Why does the ghost only like the daughter of Mr. Otis?

In The Canterville Ghost, the ghost far prefers Virginia Otis to the rest of her family. The main reason is that Virginia is gentle, compassionate, and good-natured.
In chapter 5, the ghost tells Virginia that she is much nicer than the rest of her "horrid, rude, vulgar, dishonest family." The ghost also appreciates the fact that Virginia is not prone to playing pranks on him.
Virginia's gentleness, however, is coupled with courage. She shows no fear in confronting the ghost about stealing her paint to manufacture the bloodstain on the library floor. The ghost is momentarily chastened, of course, but he is ultimately unfazed by Virginia's criticism of his actions. He soon confesses to Virginia that he has not slept for three hundred years and that he is miserable.
Virginia sympathizes with him, which comforts him. The ghost maintains that the only way he can be at rest is for a "sweet and good and gentle" child to open the portals of death for him. Since Virginia is that child, she is likely the only one whose prayers to the angel of death will prevail. Virginia helps him, of course, and the ghost receives his rest.
The ghost likes Virginia Otis more than the rest of her family because of her gentleness, compassion, courage, and purity of heart.