What Is Harley to Joker?
I speculate that Joker's response towards Harley is as follows:
A Toy: Joker sees Harley as his creation, and one of his most special ones. He broke Harley apart and remade her into his own image and continues to manipulate and bend her to his will, delighting in watching her strive to please him. When Joker is bored, he can play games with Harley and she will always play back, willingly and with pleasure. He can hurt her and draw her back to him with well-chosen words and deeds. He knows that every time Harley makes an effort to break away it is simply a matter of twisting the screws in the precise way to have her grovelling once more.
A Joke: Harley is a joke, and a wonderfully played one. He took a beautiful, successful young psychiatrist and coaxed her into sacrificing everything she had for him. He broke down her sanity, her self-respect and her independence leaving in its wake a completely insane, dependent and madly devoted slave. Her life as Harley Quinn as opposed to Dr. Harleen Quinzel would be hilarious to The Joker, who delights in such perversion and would take great delight in a promising life so squandered.
A Mirror: Harley, in both her insane devotion and her adoration of him, is a mirror to The Joker's ego. Being highly narcissistic, Harley would provide proof to him of his own genius and brilliance; both in the very fact of her existence and because she willingly tells him so and fervently believes it herself. Similar to above, a Joker in a funk could take one look at Harley and start chuckling again as she stares at him with wide, adoring doe-eyes. Harley is his; body and soul, and that must be immensely gratifying to someone as egotistical as The Joker, especially considering the ease with which he controls her.
A Loved One: Yes, I believe it. I think Joker loves Harley. Not in any self-sacrificing or selfless way. Not in a way in which her wellbeing and happiness are important to him. Not in any normal, healthy way. But in his way, as much as he is capable of. Some fans feel that “love” is too strong a word for The Joker. Nonetheless, that he does feel some sort of affectionate emotion towards him. Her uniqueness in response to him, I believe, penetrated something very deep and buried within Joker's psyche and brought it back to life; a faint glimmer of tenderness consumed by layers of darkness and depravity.