I think Cowl and Kumori are John Marcone and Helen Beckitt.

There is little evidence for who Cowl and Kumori are so I have no sources or hard evidence. The reveal for them is going to be a big part of the end books in my opinion. The rhythm of the narutive is how I came to this guess. That and talking on reddit. So here are my clues.

• John and Helen both appear in book one. The reveal at the end would be cool.
• The answer to most plot holes is as single time loop. John and Helen to the end then back to the beginning as Cowl and Kumori rethrough to the end.
• John and Helen both were devastated by what happened to Amanda Beckitt.
• John wants to protect innocent bystanders from his business. Because of Amanda but also it's good for business.
• John has already tried to save Amanda using supernatural methods, with the shroud.
• Cowl and Kumori want to stop death, or at least Kumori.
• Helen is working for John as of White Knight. She can use magic. But she doesn't know Amanda is alive yet.
• Anyone can learn magic(WoJ). John can learn magic and can gain power. By using all the ways Harry has been tempted with and one more. He could make a deal with outsiders.

"Vittorio has been given power. That is how he can do this. He is possessed.”

I wished I could have raised my eyebrows.

Possessed by what? “An Outsider,” Lasciel said.
(White Night, Chapter 41, The Dresden Files, Book 9)
{Vittorio was working with Cowl and the Cercle(Black Councle) to stop the white cort from negotiating a truce.}
   

• Cowl is working with Outsiders. This is obvious as of now. He is also a member of the Cercle(Black Councle). There goals are unclear but they have been a major player in the story so far. Helping the red cort, attacking the white cort, infecting Leanansidhe, and presumably behind the attack in Cold Days.
• Time travel is going to be a part of the story. We know Mirror Mirror will be about a parallel reality(WoJ). Other than that it has been implied subtly that Harry can do it and should be able to already. Maybe he already has come back from the future off camera.

“Perhaps,” he murmured, “you already have.”
(Cold Days The Dresden Files, Book 14) (p. 212 ch.21). Penguin Publishing Group. Kindle Edition.

“Someone is bending time against us,” Kringle said.
(Cold Days, The Dresden Files, Book 14) (p. 408 ch. 42).

 “One of the Queens,” I muttered. “Or someone operating on their level. Can we get out of this wave?”

The Erlking and Kringle traded a look. “You are the leader of the Hunt,” Kringle said. “What you wright with your power will grace each of us. Would you like to do it?”
(Cold Days, The Dresden Files, Book 14) (p. 409 ch. 42). Penguin Publishing Group. Kindle Edition.

• Some people think Cowl is Harry from the future. John and Cowl have been seen in the same place at the same time. Harry would have noticed any power from John if he was a practitioner, but John is the one of the most magic savvy vanila mortles(WoJ). It is possible in the future John and Helen will make a deal with Outsiders for power to heal Amanda, but she dies anyway so they go back in time and become Cowl and Kumori. Or Cowl sends them back in time, as a last ditch effort, after being stopped by Harry. In other words John is just John until Cowl dies then John becomes Cowl, and the same for Helen.
• Nicodemus is supposed to start the apocalypse in Stars and Stones. I think his plan is to go back in time and at the same time open a portal to Hell. His presint self will go into the past, this would explain why he talks about time being relative and his confidence. He saw fucher self go into Hell so he knows he will succeed. I think Marcone and Helen follow Nick into the past.

So that is my hole theory. Harry recently got the real shroud out of the vault Marcone was protecting. It would surprise me if John does not try to use it like last time, but he might not know Harry has it. The shroud would be a useful in bargaining for help in Peace Talks, but if Harry tries to stop in the mayhem and use it that could git Amanda killed. I don't think that is likely though. Healing will more likely come at the end after the chaos is over, but it might not work again. Cowl is most likely going to make a move to stop Peace Talks like he did in White Knight, but stopping death for every one would be a grand finale move in the last books. Cowl will probably try to do another kind of ascension move absorbing whatever big monster shows up. So, what do you all think? Is my theory too crazy?