Hume uses Cleanthe as his mouthpiece, ostensibly to relieve Philo of the task of attacking the plethora of religious doctrines. By using Philo in this way, Hume avoids personally alienating his readers. There is little doubt, however, that Cleanthes is espousing Hume's own positions on doctrine.
As for Demea, Cleanthes calls him a 'mystic'. Demea puts forth cosmological arguments for existence of God. Philo, and by extension, Hume, is skeptical of such arguments.