The question of whether or not William is a host is a tricky one. During season one and two, he appears to be human, but the end of season two makes us think otherwise. In a postcredits scene, William enters the once-flooded Forge and is greeted by his late daughter Emily. After a telling conversation between the two, it becomes clear that William is being tested for fidelity, much like how he tested his father-in-law's host. Seeing that the postcredits scene appears to be in the future, it's likely that William eventually transfers his human consciousness into a host body.