First. Concept of spirit in group seems to me lore unfriendly. Too many questions how? How he could walk so long out of fade? If he demon, he must drink someone life, or what? If he is a spirit, then he must be in someone's body like Wynne or Anders. But we already had two spirit possessed companions. How other party members would react on Cole? Some party members might think it is too dangerous creature to let Cole live. And why would any fade creature help to close portals? Too complicated to explain, too dangerous to trust him.
Well firstly, I assume you've read Asunder - the novel gave hints as to what Cole is and he can exist in the real world. The general consensus seems to be that he is a spirit/demon who inadvertently took on the identity of a dead Mage and unknowingly used blood from the Mages he killed to remain in the waking world.
This may be right. It may also be wrong.
In my opinion, a spirit/demon could work as a companion - in fact, the sheer mystery of Cole's nature seems like a good argument for his inclusion. He would be something entirely different, and could offer a whole new perspective on the Fade.
In terms of how party members will react to him - well, probably in many different ways. In past games you threw together a haphazard group of companions, some liked each other, some detested each other, but they all fought for a common cause.
Second. If it is really Cole, his in game model looks awesome, attractive. Why give people attractive and interesting character and then cut off any feelings? It is like create a super sexy Sandal, Player will come and say something like " Hey love, want to chat a little " And hear famous "Enchantment!" Now imagine that player face
Different people find different things "attractive". I am certain that Cole's inclusion has nothing to do with how attractive he may or may not look, and everything to do with his character, skills and story being interesting and important.
Third. Much easier to create character who will remind some other popular person, but more fin in current situation. No need to explain all that Cole stuff to people who never read a book. " Hello, I'm mage hunter from Tevinter" Seems much more simple than "Hello I'm demon spirit from a fade in a boy body with mystic dagger" Bioware have a lot of followers to write and limited time and budget.
I'm not particularly fond of including book/comic/film characters in the games, but I find Cole too interesting to argue against.
Yes, Bioware could have come up with another character who requires less explanation, but would that character have been as interesting? I trust Bioware to write their characters well, and I don't think I'll be disappointed.
TL;DR - There is no official confirmation that Cole is even in the game - he may or may not be the 'unknown rogue' - but, in my opinion, he would be an excellent addition to DAI, and I hope one day we do get to find out more about him.