Thank you Snark Knight, for addressing a few of those points. As SK says, Harry is only observant when it suits the plot. Harry completely failed to notice Molly being in love with him, or that the Grey Lady was Molly (and Leah for that matter), he failed to recognise Lasciel's return in Skin Game and he didn't figure out the Kravos/Nightmare situation until it was way too late - and many more besides. Frequently he only figures these things out at the worst time and only survives by the skin of his teeth. He may eventually figure them out, but I think he has a long way to go. They haven't shown up for a while and I'd say we are well overdue for seeing them so likely another encounter will be in the coming book.
I will do my best to address your points individually:
Groinkick - Very true there is a big difference in that respect, and yes I do feel that Jim wouldn't go into such dark territory normally. However even if that part of my theory is wrong, I believe I am close. Perhaps not twins by blood, but by ritual or some such. I think their connection is a deep one.
Con - What major plot hole? To your other point, I think it is still reasonable after 10 years roughly that Cowl (if he were Justin) would be interested to see how powerful his former student had become. He also acts rather like a disappointed teacher when Harry proves to be such an unworthy opponent. Justin would be familiar with the Wizard that Harry was at 16, not at 26 (by which point Harry head defeated several dangerous sorcerers, minor demons, started a war with the Red Court and killed one of their nobles, stopped a war between the faerie courts and killed a faerie queen - no minor act considering it hadn't happened for almost 1000 years, and defeated several Denarians). I think he was very interested to see how powerful his former apprentice had become. I had not read that information from the Paranet Papers, which I think is interesting however not evidence enough to exclude Justin or make Simon a more likely Cowl. I was persuaded by the Simon argument for a while but I feel that the theory does not fit the facts as neatly as the Justin Dumorne theory.
Maz - I have answered the first part of your comment above. I am confused somewhat by the fact that you believe that because Harry was a few feet from Kumori he should have recognised her. We know that Cowl and Kumori have gone to great lengths to protect their identities through both physical disguise and magic, why then would Harry the Brawler be able to see through their artifice? They both are likely better at that subtle stuff than Harry is and Justin and Elaine both know how Harry thinks, meaning they could tailor it specifically so that he would personally find it difficult to recognise them. And if Faith or Jenny, why did not recognise them? According to your argument he surely should have recognised them by their voice, smell, magic etc. No, I suspect they are not likely candidates. If you were Jim and you wanted to torture Harry (and by extension his readers) who would you pick to make the most damaging impact? Elaine. He knows Elaine intimately, as you say (though the psychic communication is a spell using neuromancy, he could choose to do it with anyone but it is borderline illegal and he invented it with Elaine and they supposedly trust each other) so the pain of her betrayal will be worse than Faith (who he has met once) or Jenny (who he didn't know anywhere near as well).
Peregrine - A reasonable argument, sure. But being "fairly confident" about something doth buttered no parsnips. Harry is constantly reassessing his history and what he believed. He thought he had no family, he thought his mother and likely his father (malcolm) died of natural causes, he thought Eb was just a nice old wizard who raised him well, he thought Thomas was a terrible monster, he thought Kincaid was human, he thought Leah was out to kill him, he though HWWBH was just some demon (not a major Outsider) etc. What Harry was sure of is only mildly trustworthy at best (especially when it comes to events in his past), not to mention things is only "fairly confident" of. The Name argument is a strong one and my counter is two-fold. One, he wouldn't need to know all of who she is to get her Name right. We are constantly reminded in the earlier books the danger of speaking your Name to another person. You don't need to know exactly what the fish looks like in order to spear it, only to see enough of it and have a good enough method to catch it. A clumsy analogy perhaps, but I am a bit pressed for time. Two, JB unfortunately retcons and creates plot holes all over the shop. I suspect this could be one of them if it came down to it.
morriswalters - SK mostly answered your comment for me. Justin teaches and learned in the school of hard knocks, there are many memories Harry has of his harsh lessons. I suspect he expects everyone to be capable on their own merits. In that particular scenario, one should note Kumori and Elaine are not as major brawlers as other evocation specialists and that the Skavis snuck up on her, which is the bane of every Wizard as JB keeps reiterating. Baby Bickett took me a moment, did you mean the injured Beckett child that Marcone feels responsible for? I mean it is possible but that child wouldn't affect Harry on the personal level someone close to him like Elaine or Susan or Murphy would...it is more a way to get at Marcone if anything. I mean the whole Beckett child thing is not something Harry felt responsible for - he just is sad that it happened.
As for the sword, you must remember how uncommon it is that Wizards fight in direct combat, let alone duels. Harry is definitely an outlier compared to the majority of Wizards and even Wardens. The only reason you see more guns amongst the Wardens is that so many of the old guard were killed and Harry was by far and away leading that pack of new guards carrying modern weapons. Harry having a sword cane that he would use, in any fight, is just plain odd by most standards. Especially as he had it long before he was a Warden. Even only one other Wizard who wasn't a Warden carried a sword, and that was the Corpsetaker. And we really don't know much of his past. Luccio carries a sword not merely as as a secondary combat tool, but as a symbol of office and as an anti-magic tool. Yes it was popular with her generation and with most generations right up until the mid 1900s, but mostly the end of the 1800s. Death by magic (to human mortals) is indeed black magic, but Wardens don't just exist to kill Warlocks. That was mostly Harry's (somewhat justified) prejudices. They do spend a lot of time fighting monsters of the more-than-mortal sort, more so really than they do hunt down Warlocks.
When Mab referred to Death as a spectrum, she was not specifically or even indirectly referring to Necromancy and its ability to bring the dead back to life. I suspect the reason they did not simply raise Bone Tony is that having already tortured and killed him, Grevane arrogantly assumed that he had no more useful information. They picked on Butters as they assumed he might have knowledge that they had missed. Kumori's inability to heal a man who had crossed over does not indicate that could not do so under other circumstances, or that another could not do so using Necromancy. She already had pulled off a feat normally reserved for Gods and other similarly powerful beings. What I believe Mab was referring to was the fact that Mortals believe Death to be the end of existence, which is only true up to a point. By all logic in the Dresden Files, once you die you move on to whatever afterlife awaits you which is another form of existence. True oblivion (where you do not exist at all and are not conscious in anyway) might only exist in certain circumstances, but merely dying in the Mortal plane does not take you there. Perhaps some beings mortals and immortals alike might not have an afterlife (e.g. vampires, gods, demons, faeries and animals) but we have no knowledge of that in the Dresden Files. To answer your last bit about Death Curses - who says he needed to verbalise the Curse? By all accounts only by killing a Wizard before they are aware they are dying (which includes Grevane, who simply refused to recognise he was dying) such as using almost instaneous methods allows you to avoid them. Having been burnt myself, and having witnessed some terrible burning incidents in the line of duty I can tell you that while people freak out, unfortunately you can still think. You are not simply aware of the pain - you are aware of the danger and you are conscious enough to try and take action. It is all the more terrifying because you are aware. Besides, despite how many achievements Harry has achieved killing a highly experienced warden with fire magic because he was scared seems implausible. Justin had survived many engagements and as a Warden would have been very capable of fighting. He may have been caught off guard, but he might have also diverted his remaining strength into surviving. He could have opened a Way or switched bodies - most survivors tend to be the ones that rabbit earlier rather than later. If you wait too long you die. End of.
A few interesting things I would like to point out - Cowl has scarred arms. Scars from what? Burns perhaps? Hell of a feature just to throw in there about Cowl. JB is a lazy writer remember.
Also when Harry talks to his Id - normally there is a secondary message that become relevant later (headaches, lasciel, being too closed off etc). In Dead Beat, he discusses Dumorne with his Id several times. It would fit JB's style to an extent to lay the foundations for Cowl's reveal in the first book he is introduced.
Finally, I really appreciate the debate from all of you. I enjoy testing the strength of my arguments and hearing others opinions. I would love though if someone can rebut anything or provide an alternative to the points I mentioned better - which some of you have but on balance most of my arguments for their identities remain intact. There has not yet been a strong enough counter-argument or counter-theory that I think rules out my theory as to the identities of Cowl and Kumori. But I am sure someone will have a go and I look forward to the challenge!
