The Son is basically an embodiment of the dark side of the force and being its most prominent and notable embodiment, he can be classified as the devil of the Star Wars universe. He is a powerful shapeshifter and manipulator who aided in ushering Anakin Skywalker's first steps into the dark side, he even managed to possess Ahsoka Tano with his influence for a short period of time. Despite his vast power, his mental state was unbalanced and tormented and after he unintentionally slayed his sister, the embodiment of the light side of the force, he became more unstable. Because of his unstable emotional state of mind, he would eventually be killed by the chosen one, Anakin Skywalker. It is unknown if the dark side ever gained another embodiment after his demise.
Although he's power hungry, manipulative and treacherous, he loved his father and sister, as he expresses his sorrow, when he met his dead sister and when he saw his father was dying. Because of his conflicting emotions, he often appeared emotionally and mentally unstable.