B.G. Grantham is a physicist who alternates periods of intense mental labor with periods of intense sexuality. He has a fabulous estate and a best friend, Eric Berne, to help him run things. B.G. and Eric have a loving sexual relationship, but B.G. is sure Eric needs a wife, because he knows he doesn't provide enough for his friend. Enter Charity Wills, who is offered the chance to become their sexual "guest" in exchange for college tuition.
Charity has the gift of putting people at their ease and seeing what they truly want (up to a point--she's not perfect). She has a crush on Eric long before she's invited to be his sex partner, we're shown enough of Eric and B.G.'s past relationship to be rooting for them as well. I especially liked that Charity found it really hot to watch Eric and B.G. together. Slash in the mainstream!
The one character whose presence felt forced was Sylvia, who provides Conflict of a sort. She might have been required as far as plot-generation, but I would rather have done without her.