Since that day, Harley has been on a mad rollercoaster of obsessive love with The Joker. She has been in and out of the Asylum, both as an escapee and as a rehabilitated patient, over and over again.
Her infatuated love for The Joker dominates her life; he is her motivation to the life of crime which she leads in order to remain close to him. Almost all of her actions are done with his happiness in mind. She is terrified of him and insanely in love with him and can barely imagine a life without him. Her sick infatuation leads her to carry out his every beck and call and she is cowed by him and utterly adoring of him.
Their relationship is a complex one, following a similar pattern to the typical abusive relationship. Joker physically and psychologically assaults her, then draws her back in with comfort and affection. She makes the vow to walk away, only to find he is a drug she cannot be rid of. Her attempts to stand up for herself are eventually dismantled by either his threats, or her fear of losing him.
With someone as insane as The Joker, his reason for keeping Harley around instead of killing her are uncertain. Almost definitely, he views her as a 'toy' - something he created, and most likely he values this; his ability to reduce an independent, attractive and successful young woman to a willing slave who has given up all hope of a normal life for him. Doubtless her undying adoration holds amusement for him; and is a reflection to him of his own ego.
But it is also very possible that Harley truly has touched Joker in some way. The Joker is renowned amongst all as being one of the most dangerous and unpredictable maniacs the world has ever seen, generally accepted as loving no one but himself - as not even seeing other people as 'real', but merely as objects to play with. Yet it seems that somehow, Harley has penetrated through the twisted paths of insanity to touch some long-forgotten core deep within him.
For her part, Harley will never truly be free of her love for Joker. She is utterly infatuated and will remain so, even during her periods of 'sanity'. He has completely bent her mind to his and changed her life for ever.
Most of the time she's pretty happy about it.
Harley has a vivacious, effusive and highly expressive personality, quick to show affection for her loved ones, just as quick to anger and snap. She is a woman of extremes in all things. Her mood swings, whilst nowhere near as violent as Joker's, are still mercurial to the extreme. She experiences all emotions intensely and passionately.
She is undyingly loyal to her two best loved ones - Joker and Poison Ivy - and often ends up as a bit of a 'door mat' to both of them, being cowed by their more dominant personalities.
Whilst Harley seems merely zanily playful, she is absolutely insane, unpredictable and therefore highly dangerous. Her love for The Joker makes her fiercely defensive and protective of him and she will go nuts in order to do so, or to defend their love to anyone who questions it. She will do whatever it takes to help his dreams come to fruition - as mentioned above, she's a highly determined and fixated woman - as well as pursue her own goals with the same fervour.
She is possessive and jealous of both Poison Ivy and The Joker.
It is speculated that she and Ivy have something of a romantic and sexual element to their relationship. Certainly Ivy would be more willing to give her the affection Joker withholds in order to control her.
Harley believes that The Batman is the source of Joker's pain and hates him as a consequence. This makes her a deadly enemy to the Dark Knight, none more so than when she is fighting by the side of her man.