Who is the guy with an English accent in the cell at Demonreach?

[peregrine]

Maybe Jim doesn't necessarily think every little thing through all the way, and sometimes makes mistakes.

[Arjan]

Maybe Jim doesn't necessarily think every little thing through all the way, and sometimes makes mistakes.

Blasphemy!

Besides even if it was a mistake he will make it fit into the story.

[vultur]

He might be somebody from real life/folklore from England who we haven't yet heard discussed in the Dresdenverse, somebody associated with weird cosmic horrors...

What about Dr. John Dee? In real life he was a scientist/philosopher and advisor to Elizabeth I who promoted the development of a British empire, but Lovecraft associated him with the Necronomicon -- which might suggest a reason for him to be in Demonreach. It's also interesting that (according to Wikipedia, anyway) both the record of his date of death and his gravestone are missing! Hmmm....

[Kindler]

I like the theory that it's Chandler/Steed. WOJ is that his specialty is time-screwery. If there is ANY time travel in the series, my bet is that Chandler will have something to do with it. I've toyed around with the idea that Steed and Harry go back in time, and for one reason or another, Steed can only send Harry back to the present, while Chandler must wait it out. That's why he only talks about "stasis" rather than imprisonment; he's waiting until he catches up with the present so he can step into current Steed's place. Living outside of stasis would present far too great a threat to the current timeline, as it's possible for Steed to change things in such a way that he never goes back in time in the first place, which would create a nasty paradox.

Either that or it's Arthur, in my opinion.

[groinkick]

Wonder of Rashid put him in there....

[spiritofair]

If it is Merlin, Merlin is a known time traveler (he built demonreach across multiple timelines, right?) so he could probably speak with a modern English accent.

[segaily]

Working on the assumption that Butcher doesn't throw out random noise, why does he point at that particular prisoner?  Is he perhaps the original Merlin?  Or someone else of importance?

Merlin was who I have always considered as the likely suspect.  He could well have put himself there as the final fail-safe if the prisoners ever got released and or could know he was going to be needed to save the world in the future. 

[Wizard Sibelis]

...Unless he did as has been spoken of and changed it because it was guessed..., I'll continue to guess it was Gawaine.

[Arjan]

...Unless he did as has been spoken of and changed it because it was guessed..., I'll continue to guess it was Gawaine.

My cat has nothing to do with that mess. Really, I asked.

[groinkick]

...Unless he did as has been spoken of and changed it because it was guessed..., I'll continue to guess it was Gawaine.

Why Gawaine?  Wasn't Arthur mortally wounded and taken to an Island to heal?  I'd think it was Arthur brought there by Mab to be kept until a later date when needed.

[Wizard Sibelis]

Why Gawaine?  Wasn't Arthur mortally wounded and taken to an Island to heal?  I'd think it was Arthur brought there by Mab to be kept until a later date when needed.

Yea, but Gawaine would make an excellent Warden of a previous era. (I'm not looking for Arthur because I'm under the assumption he's been found in one place or another, mainly, in EK and similarly deified Gods of death, or Mac, Mac could totally qualify as the original)His green-grey cloak(implying a certain in between nature and summery connection), his pentacle shield. It's based off of the resonance of the five points repeating just like DR is, same magic layering..? Plus a certain cluebat woj about if a wizard were crazy enough he could ground magic out using a shield, that 'craziness' lends to his later need to stay in DR, his ability to extend a perfect circle of will and project it forward to surround and contain a creature while you maneuver it... would, I think, lend itself quite well to a warden capturing things on DR.

[SerScot]

its James Marster being punished for not being available to do the voice work for the audio Cold days.

James Marster read the audiobook for Cold Days

[RobReece]

As far as Merlin is concerned, Jim has on multiple occasions stated that the British prisoner is not Merlin.
But, the caveat being Jim doesn't always tell the truth when relating to future plot points.

[Quantus]

We dont know who it is.  We have two WOJ regarding Who it is not: it is not the original Merlin, who would not sound at all british he says, and it is not Loki who is suffering the very literal Norse punishment of venom dripping in his eyes. 

2015 San Francisco signing (Coopersfield Books)
The original Merlin, does he sound British?
He’d probably sound so unintelligibly British that you wouldn’t be able to tell he was speaking English.  No, he’s not the guy in Demonreach.

2015 DragonCon
Is Loki in demonreach?
He is not in Demonreach. There are no snakes dripping venom in there, and Norse gods are awfully literal about that sort of thing.  I don't know if we will hear from him or not... Well by the end, we are going to get Wagnerian by the end.  Wagnerian for you opera fans.

[Snark Knight]

We dont know who it is.  We have two WOJ regarding Who it is not: it is not the original Merlin, who would not sound at all british he says, and it is not Loki who is suffering the very literal Norse punishment of venom dripping in his eyes.

Given that Jim has also said he'll lie in Q&A if necessary to prevent a spoiler - and "I'm not going to tell you" in reply to asking if the prisoner is the original Merlin would pretty much be a confirmation - I'm putting limited weight on that one.