Tim Drake is the Third robin in the comics (after Dick Grayson and Jason Todd) or the second Robin in the animated serie (in Batman Animated, Jason Todd doesn't exist). In the movie "Batman Beyond: Return of the Joker", it related that Joker an Harley kidnapped and tortured him, to make him their child, somehow, and do a little Joker. Image: [link]
Sigh. You have no idea about their couple, friend. None at all, what you said has no actual ground but your subjective views or ignorance. Read up on actual comics.
He considers her his girlfriend, he openly admits it, in a number of comics including famous Mad Love and, according to the author Paul Dini, who created Harley, he does love her to the best of his abilities as a psychopath.