I've been re-reading passages from Asunder recently, trying to recall Cole's character, and I admit, I'm fairly puzzled: it's practically impossible to make him into sympathetic character, let alone a companion.
- In the book, he is basically an evil serial killer. In one of the beginning scene, he kills a helpless girl, and does it, I admit, in a fairly disturbing(if not disgusting) manner, stroking the victim's cheek, speaking of deliverance and so on(and then he kills her). He kills again, and again, until any sympathy anyone could feel towards him finally nears annihilation.
- He is a demon, and Seeker Lambert, I believe, is very close to the truth when he cries out that Rhys could become his next host. Anyone could become his next host - that's what demons do. He preys on people, he kills people, and simple common sense begs one to start reading the Litany, not to take him to the party. (And we all remember the story of Justice and Vengeance - though, privately, I think Anders wasn't entirely wrong, given how the Chantry wanted to destroy the entire city in their holy war).
- He also kills a fairly sympathetic character - Lord Seeker. To me, Lord Seeker is very-well written, is a dedicated character, and noble, too, in a way. Basically, Lambert is a three-dimensional Meredith with a strong character and understandable convictions. I certainly sympathize with him, and though I usually support mages, I cannot side with Rhys in the book. Rhys' speech "nooo, Cole is good, there's so much we don't know, come on, he just killed a few random innocents, who cares?" seemed hollow next to Lambert's calm reasoning. Seriously, I'd rather have Seeker Lambert as a member of the Inquisition than Cole.
- even as Cole tries to be "good", he continues to rape and violate the minds of people by erasing their memories. Imagine that someone helps you, and then he suddenly kills the _you_ he helped: you've been thinking a thought, and this thought is erased with your consciousness; in your place now stands another you, without your last memories and insight. Your consciousness has been deleted, your memories of the last hour are gone; another entity stands in your place. It might be 99,9999% you, but still, not quite. That's Cole's most horrible trait - and judging by the quotes, he doesn't seem to be ashamed, he seemed to be proud that he "helped" people in this perverse way. And he intends to continue.
So, I can picture Cole as a very dangerous antagonist we certainly need to kill, a demon we need to destroy, someone who understandably causes the feelings of shock and perhaps even hatred - a creature that causes that much harm and suffering should be stopped, certainly imprisoned.
But a companion? A friend? Well, I do play evil characters, I play murderous characters, but I think even my Sith Warrior would hesitate before asking Cole to join the team.