Their relationship is abusive. While the Joker is clearly the primary aggressor, Harley is also shown to be capable of violence and manipulation herself.
The relationship is unbalanced. The Joker manipulates and devalues Harley to maintain control over her, and she seeks his approval and validation even though he treats her poorly.
Finally, the relationship is codependent. They are mutually dependent on each other for emotional and psychological support, and they enable each other's destructive behaviors.