Though their relationship seems to have been retconned in the new 52 universe, almost everybody familiar with the characters knows that Harley Quinn and Poison Ivy are often considered to be best friends.
They often are seen teaming up not to achieve some higher goals but due to a geniune friendship.
This got me thinking: why is that? What is it that makes the two characters compatible?
The reason I put this in the Ivy forum is because her caring for Harley is what perplexes me the most. Harley Quinn is generally open to forming friendships and relationships more than Ivy. Whereas Ivy has little regard for human life and hasn't generally been shown to care for other humans. What is it about Harley that makes Ivy so accepting of her? Was the reason behind their friendship ever truly explored in a comic?