In my opinion a mantle must have 5 things (but I reserve the right to add more if they occur to me):
1. Power
2. Purpose
3. Limitations
4. Belief
4. Recognition
Now these five things are separate, but tied to one another and can cover some of the same areas. First comes power. A mantle must be invested with power, otherwise it is just a name. Power must be invested to create it (ie WoJ about the Erlking creating a knight from his power) and must remain with the mantle. The power does not have give the mantle holder a big boost in physical power like the winter knight mantle. It can be in the form of knowledge or responsibilities.
Number 2, mantles must have a purpose. Even if that purpose is to act as the mantle creators proxy. The purpose can be anything the mantle creator desires, but it should be defined and if disobeyed their should be consequences (Slate as Winter Knight).
Number 3, mantles must be created with a specific limitations in mind and should be revoked if necessary. That links the mantle to the one who created it and prevents the mantle holder from using the power indiscriminately. For instance, Harry can do his own things with the winter knight mantle, but he loses it if he crosses Mab or Winter Law (as seen in Cold Days). Harry also loses the mantle if he is pierced by iron. This should also theoretically keep the mantle holder from turning on the mantle creator.
Number 4, a mantle should be believed. My personal WAG states that mantles are somewhat powered by belief. That helps power the ongoing mantle for its daily actions. For instance, most people believe the Winter Knight to be Mabs enforcer, thug, assassin, etc. If Harry is acting in accordance with that he remains empowered. If he acts completely contrary to that his power is diminished. Belief by people aware of the mantle and the mantle holder can slowly change the mantle. This is Harry's hope for himself with the Winter Knight Mantle.
Finally, number 5, recognition. The mantle must be recognizable as a mantle and as belonging to a certain faction or person. The winter knight is recognized as a part of winter and must act in accordance with winter. If the Erlking created a mantle it would be associated with him and his interests and would lose power if it diverged.
As a final note, I use all of these to support my opinion that The Warden of Demonreach is a mantle held by Harry.
