Cowl is an unknown, extremely powerful wizard, who is power-hungry, but sane. He may actually be one of the wizards on the White Council, or someone entirely different. His face has never been seen, and his real name is unknown.
Cowl and Kumori were dressed nearly identically in long black robes, long black cape, big black mantles, big black hoods which revealed nothing of their faces.
Cowl can appear to be something he's not—he came up behind Harry as a zombie lifting his spear, then shimmered into his robed self and threw out a spell at Carlos Ramirez that threw him twenty feet. Then he quickly blocks a Hellfire-strengthend kinetic blast that Harry aims at him by crossing his arms in an x-shape, he didn't get slammed but he did get put off balance.
In the SeriesEdit
In Dead Beat, Harry meets Cowl for the second time, where he's searching for Die Lied der Erlking and The Word of Kemmler with Kumori, his assistant. Harry recognized them from Bianca's Ball as the ones who Leanansidhe the Athame. Harry flips a car of him, and Cowl manages to escape with the aid of Billy Borden and the Alphas. 
Later in Dead Beat, He knew that Harry possessed Bob the skull, and took it in order to learn how to perform the Darkhallow. With Bob, he had no need for the Word of Kemmler. Cowl disappeared after the workings of the Darkhollow were disrupted.
At the end of Dead Beat, Harry tells Morgan and Listens-to-Wind that he thinks Cowl had a deal with the Reds to keep the Council busy while he performed the Darkhallow. The Reds would get the Senior Council out of the deal. Morgan thinks the Edinburgh defenses are too good. It would take a god to break the defenses and that's exactly what Cowl was planning to do with the Darkhallow.
In White Night, Cowl brought the ghouls to the White Court Coup in the Deeps, which may imply that he was also a part of the New Mexico incident at Camp Kaboom. His skills at opening up portals to the Nevernever are better than Harry's and he is also able to close other practitioner's portals from a distance. He can sense other practioners from a distance.
Other Details Edit
The spell he uses on Harry at their first encounter is; "Dorosh!" — pure will focused into a violent kinetic burst. Harry having his shield ready was the only thing that save him.
C: "I was the only one of the three of us who thought you would succeed." H: "I thought you guys hate one another's guts. Are you working together or trying to kill each other?" C: "Why yes. We smile at one another and play nicely all in the name of Kemmler's greater glory of course. But we are all planning on killing one another as soon as it's convenient."
Cowl tells Harry that the "Council is not what it once was. It's rotted from the inside, and many Wizards have chafed at its restrictions have see the War with the Red Court reveal its weakness. It will fall. Soon."