That of course would be me.
Mister at the point of Little Things is 17-18 years of age. Everyone is noting he is quite old, by the calendar, but it is clear from Little Things he is as strong and quick as he has always been, with senses to equal Toot and Lacuna. The Little Folk are noted as being sensitive to to things, Harry has to Listen to even hear them sometimes. He is also relatively peacefully co-existing with The Za Guard. If he were a Malk Changeling, like Sarissa he wouldn’t age unless he chose mortal, and he would fall within Winter and would therefore allow Toot Etc to live, relatively unharnessed. Toot has only been Winter since Changes, prior to that he would have been fair game. A mortal cat wouldn’t make the distinction, a scion would. His blood thirstiness is also a Malk trait, they are worse than mortal cats. The constant reference to Tomcat suggests Mister is not fixed (we know Mouse isn’t) and yet he is not spraying, suggesting he is intelligent enough to not do so. The Mister Microfiction gives some idea of his intelligence.
I think yes Mister was a spy originally for Lea, but as a dog person she failed to address the issue all cat lovers are aware of “Because Cat”. He has been a terrible spy because he has been pursuing his own agenda all along, he is extremely happy being what he is living a life of pets, catnip, random mayhem and sex and terrorising a much larger semi-divine dog. He is living the perfect Cat Life.