Lucifer is an important character in not only Inferis, but in Caelum. In Unspoken Gods, he is still considered an archangel, and Gabriel’s counterpart and equal. While Gabriel rules the angelic realm in Caelum, Lucifer rules Hell and runs the angelic kingdom of Inferis.
He is responsible for overseeing the entire kingdom of Hell, and assigning the punishments for souls there in order to have the best effect on them to get them closer to redemption.
In Unspoken Gods, Lucifer is not an evil figure, but rather acts as a symbol of hope for the souls trapped in his realm, meant to push those souls to work through their sins and become better so they can move into Caelum.