ChaosMorning and I are on the same page from the sounds of it. I think Cole has underwent several 'forms' as it were. I'll do them in order;
1). Cole, the human mage. A young boy, with an abusive father, a deceased mother, and a sister he accidentally smothered to death. Used his mother's dagger, a keepsake from her, to murder said father. And was then taken to the Orlesian Circle, thrown in a dungeon. He was afraid, and hoped for release.
2). Cole, the Hope Spirit Abomination. A spirit from the fade visited him in his cell. We believe it was benevolent because it was there to comfort him while he was powerless against his templar captors. This spirit was with him until the very end, much as how Wynne's Faith Spirit was there for her. Hope is apparently the strongest form of Spirit (at least 'so far encountered'), which would lead one to believe that it's next form is even more powerful than we can comprehend.
3). Cole, the Despair Demon Abomination. When the Templars were too rough on Cole and he passed away, they erased all evidence of him to hide their shame. The spirit didn't like this, it was corrupted, in a way similar to Justice becoming Vengeance. We've been told to expect new types of demons in DAI, and I thought I remembered hearing something about a Despair Demon. Cole, the boy, is an abomination. The spirit of hope is no longer. And what remains is an extremely powerful despair demon. One who, for most of the book, was unsure what he was. Cole the Human versus Cole the Abomination, both struggling for some resemblance of identity and control.
The Litany of Andrella works on him (works on demons), he's capable of extremely potent blood magic (a skill learned from demons), and in the epilogue of Asunder, he comes back and basically tells us "Cole actually existed, and was powerless. Now I'm here, I'm self-aware, and I'm not nearly as powerless." I read the book and thought "This would be an incredible companion for a future game." Like, we've had Mage Warriors, and Warrior Mages. But Cole would be one heck of a Mage Rogue. "A Spirit Assassin." He carries a dagger, does all of his killing with the dagger, and is unbelievably skilled in Blood Magic so much, that he wasn't even willingly using it. Imagine the squad dynamic he'd bring. A Self-Sustaining Blood Mage Rogue. It's marvelous, and terrifying, all in one.