It occurs to me that more than one character has shown the ability to effectively disguise/transform themselves in the series, mostly via shape-shifting but also by illusion and magical concealment.
Initially I will only examine human characters, then the supernatural.
MORTALS
Cowl and Kumori were able to conceal themselves whether by mental illusion or physical transformation (or some combination) into zombies. How well the disguise would have stood up to intense scrutiny depends on a variety of factors and is unknown. What we do know is that they were not immediately obvious to Harry, Carlos or Grevane. The Corpsetaker relied on a combination of new bodies and psychomancy, so I doubt ever had to use such methods (which isn't to say It couldn't).
Listens-to-Wind has shown to be an able shape-shifter, and it wouldn't surprise me if all the Senior Council Wizards were able to do some version of concealment/transformation e.g. Ancient Mai would use enchanted objects (as is her speciality), Rashid might have something else etc but it all amounts to the same thing. Harry often used to say the biggest difference between Wizards and Sorcerers were Wizards were able to do use the full range of magical spells and schools whereas Sorcerers were only really able to do destructive magic (mostly Evocation). This has been somewhat challenged though, as Harry often goes on about how he can barely do other magic apart from Evocation and Thaumaturgy (yet he can do most of the other stuff, in fact he has gotten better over the years).
SUPERNATURALS
Mab hid as a statue, and when we first met her looked like a beautiful yet human woman.
Lash first appears as Shiela.
Grey and his kind can appear how they wish (although they have to make an effort to conceal their eyes).
Uriel first appeared as a Janitor.
Many if not all of those beings could probably appear however they wish - not to mention in order to just show up have to condense their form into something comprehensible to mortals (like Ferrovax)
With this in mind, can we think of any times these characters (or others with the ability) might have appeared in a book but in disguise? For instance, if Cowl was Christos - Cowl was in Changes. But if Cowl was Wizard Bluebeard, then he shows up in (Summer Knight I think?).
