Jessom can be saved as Incubus to become an ally. He was the original Incubus, the painter from the legend of Incubus.
First Encounter: Forgotten Pacts - Hour of Ash - An Artist Depicts the Abbey (DLC)
He is dressed in his own unique version of Conjurer raiment and wears a beret on his head. Wether he is still an artist or not is unknown.
Defeated as archfiend: "N-No! I don't want to die!"
"What is our target?" - Beginning of Phantom Quests
Player defeats a creature: "Why kill it?"