Oh I think I stumbled across a theory that Sarissa is/was Kumori. Kinda interesting. I'd suspect if that is the case that she no longer is held by Cowl.
One of mine, indeed pretty much all the theories are mine, even where they contradict each other. Especially when they contradict each other.
Cowl would have to have a connection to Kumori and both Nameless and Sarissa were members of the Winter Court. Mab was at one time a mortal practitioner (see Peace Talks) and is mother of Sarissa, with magical ability running down the female line. So Sarissa should have had the ability to be Kumori. Sarissa’ has an inclination towards healing (as per Cold Days) and Nameless was a previous known associate of Kemmler (The Law) and would therefore have knowledge of life and death that Kemmler had, attractive to someone who wants to defeat death. Kumori uses that knowledge to save Marcone’s man on Whacker, something pretty much beyond Listens To The Wind.
Nameless would have been manipulating Sarissa for years using Lloyd Slate as his personal bogeyman to force her to seek his protection (Lloyd was bad to all the changelings see Summer Knight) and then dangle the offer of forbidden knowledge. Being expelled from Arctis Tor on suspicion of involvement in the attack on it in Proven Guilty he lost his ability to manipulate Sarissa and no more is heard of Kumori.
Kumori was involved in the events of Grave Peril (where Maeve had moved her Court to Chicago at this point), Blood Rites (she is shown abducting the puppies with Cowl in Fugitive and Zoo Day) and Dead Beat. That fits perfectly the arc of Nameless and Sarissa both being part of the Winter Court and in Chicago.
Re read The Law. It expressly points to Nameless as being connected to Kemmler, and being part of the Winter Court exiled for suspicion of involvement as the inside man in the attack on Arctis Tor. Both of these would be required for someone to be Cowl (in Dead Beat Cowl recognised Bod and that he contained The Word) He is the right power level for Cowl and resides in Chicago. As Marcone’s Lawyer he was aware of the vault in Skin Game and would have been in a position to notify Tessa to try to derail Nick, suggesting she was the Denarian involved in the attack on Arctis Tor.
Harry has some of this information The Red Cap, Sarissa and Mab all have more information. Marcone and Namshiel too will have information regarding Nameless/Tessa, nobody has it all but I suspect Harry will put it all together over the Next Book.