I think that Aradel is not human. I think that he is either a kandra or is something else.
1. Aradel showed up after being away for 10 years, conveniently after Captain Brettin "retired". TenSoon was impersonating Brettin, according to WoB.
2. In his chat with Wax, Harmony admitted he "worked hard to maneuver Marasi into a position where she could do good in this city." My theory is that Harmony instructed a kandra to impersonate Aradel, and then Aradel was in a position hire Marasi on as his assistant.
3. Aradel routinely gulped steaming cups of tea, and the heat didn't scald him. The other constables figured that he had no tastebuds. You'd have to have NO feeling of pain in order to chug scalding liquids.
(Can kandra be scalded?)
If he were a kandra, he would have to be pretty sneaky, fooling Milan at the end of SoS. That's one reason why I think he may not be a kandra but something else.