First of all I know Cowl's identity is a well worn question, but after looking around the forum I didn't see this seemingly obvious idea put forward: Odin is Cowl.
So who is cowl and what do we know about him? He's a dude named for his attire, a cowl. His magic "feels" dark and powerful but mostly mortal. All the direct actions we've seen him take are doing one of three things:
1. Killing people or trying to kill people.
2. Stealing power and knowledge.
3. Manipulating the politics of various nations.
Who is Odin and what do we know about him? He's a dude nicknamed after his attire, a cowl (One of Odin's most famous names is Grimnir, from the Norse story Grimnismal. Grimnir means "hooded one". Dude's nickname is literally Cowl). In the dresdenverse he used to be a God, but became killable to stay involved in the mortal world (WoJ), so presumably his magic is "mostly mortal". All the things we've seen Odin do (and the things he does on 99% of the stories about him) are one of three things:
1. Killing people or trying to kill people (and titans).
2. Stealing power and knowledge (in the sagas Odin has like 5 separate stories about stealing power and knowledge, and he kills people in most of them. In one, mead flies out of his butt, at least according to Niel Gaiman.
3. Manipulating the politics of various nations.
Jim has said that some of the characters we think are nice are gonna wind up as traitors. That was pre-BG, so he could have meant Justine, but I'm always asking myself why one of the most sinister explicitly "black wizard" figures in mythology is portrayed as a really nice helpful dude in dresden files. Odin will seduce you then murder you because he wants to steal your dad's stuff. Dude is literally a necromancer. He isn't a nice guy. Also starwars fan Jim chose to use "Vader" in Odin's public persona.
A couple times Jim has said that giving readers clues is tricky, because sometimes he thinks things are super obvious, but reads miss them. I wonder if having a black wizard named "hooded one" out of mythology and having him disguise himself as a black wizard named "hood" is one of those.
"But wait," I hear you say, "Odin claims that he and Uriel have lunch once a year, and One Eye's use of the Mr. Sunshine nickname supports the idea. Odin could be lying, but I'd he's telling the truth it implies Uriel would associate with an evil power!" Mab. Uriel worked with Mab. If you believe Bob, the archangels have even worked with Lucifer post-fall on Job. I admit that his association with Uriel is a big flaw, but it's not inconceivable to me that either the angels are more old testament than dresden realizes, or Dresden drawing a line between the Black Council and Nemesis is in error, the way Rashid's take in TC was.