In some ways, the two characters could not be more different — one is an unpredictable antihero with a thirst for vengeance, the other is a childlike extraterrestrial plant like organism with an extremely limited vocabulary.
It seems like Harley might find Groot fascinating — and Groot might benefit from the change of pace he’d find in Harley’s presence.
While on the surface, a Harley Quinn and Groot meeting may seem like a bizarre choice, there is a lot of room for great storytelling.