So my favourite theory is that Cowl is future Harry, but recently I have been entertaining another idea:

Cowl is time traveller Billy and Kumori is his daughter.

So what leads to Billy becoming a dark overlord?

During one of the next Books (Peace Talks or the one after Mirror Mirror) Georgia dies. Billy is devastated and starts researching for ways to bring back the dead. In the last case file Harry defeats Cowl and recognises Billy. He hurries to Billy to stop him from becoming Cowl, but, when Harry frantically knock on his door, Billy panics and time walks away with his daughter.


So indications to this theory:

• During Dead Beat when the Alphas approach Cowl runs away. This isn't because he is afraid of them, but he is afraid of a time paradox or at least Georgia recognising his smell
• Additional during this scene Cowl chuckles about Harry calling him Darth Bathrobe. Billy is probably a fan of Harry's Humor.
• After the Darkhollow blasting of in his face he was probably badly hurt. Billy is shown to have started developing self healing. He is the only character we have seen doing this on screen till now. (Although LTW probably can do this too.)
• When first see the Alphas Billy proposes a plan to go ahead and kill the FBI Agents. He is introduced as liking the blacker solutions, but Georgia is his backbone and moral compass
• Kumori probably never really gets to know Harry.
  So in Dead Beat she is curious about the guy her Dad told her so much about.
• From narrative point of view this allows us to view Cowl's slow descent
• Also from the narrative point of view, it us smart to introduce the main antagonist in the second book. The loup garou might get relevant again but I doubt he is a villain. That only leaves the Alphas.

So feel free to tear into this theory.