Joker’s own power is very probably an attraction also for Harley, as well as knowing she is the only person to be so close to him, to have seen the private face the world does not. To have survived so long at the side of so deranged and dangerous an individual, as well as witnessing his power over those around him, probably gives Harley a sense of power of her own, however much an illusion it is. This would be as much a drug as Joker himself or the feeling she has for him, especially to feel that she is the one who is by his side – and the one to have succeeded in getting so close.
