Having caught this couple on camera, a number of questions came to mind. What species? Which one is the female? Well, the answers are in this wonderful blog: Butterfly of Singapore.
Euploea is a genus of milkweed butterflies. The species are generally dark in coloration, often quite blackish, for which reason they are commonly called crows. As usual for their subfamily, they are poisonous due to feeding on milkweeds and other toxic plants as caterpillars. The latter are aposematically colored to warn off predators from eating them, and the adult butterflies are often mimicked by unrelated species which are not or less poisonous.