I like to think it was a combination of both - the Master set out to find a sucker to play the part of his wife because politicians are expected to be married, and stuff, but then sorta fell for Lucy, just a little bit, in a strange mad-crazy kind of way - kindred spirits, so to speak. ;)

That's not to say that he won't throw her to the wolves, should circumstances require it, of course. After all, he's the Master. :)