IIRC, in Ghost Story, Uriel addressed Mouse as "Little Brother" - at least in the context here, I see that as indicating Mouse to be an entity on similar to Uriel, lesser, restricted by rules/mortal bodily manifestation, but definitely able to interact with (aka, attack/hurt/harm/destroy) Anduriel.
Quote: "We* went through the door and were greeted by a low, warning rumble. A great mound of shaggy fur, lying beside the room's single, twin bed, rose to its feet." Ghost Story, HC, pg. 467.
* the "we" is Uriel and Harry, operating in a certainly "spiritual/angelic" state of existence, differing from a "ghostly" plane of existence, yet one that Mouse clearly sees and operates on, and imo also the same plane of existence that Andurial exists on- at first, Mouse is warning the intruders with his growl, before he recognizes them. (possibly a hint of the kind of protection Maggie needs or might end up needing?)
Quote: " "What has begun must finish, little brother," Uriel said. "Your task here is not yet over." " ibid.
Uriel wouldn't use that form of address if it weren't truth. Also (as a side-note), Mouse is communicating with Uriel, but Harry still can't hear what he is saying, even though he gets to hear Uriel's answer.