Fleet is the Genius loci of the River Fleet. She is the daughter of Mama Thames. She looks like she's in her 30's, and is said to be built like a sprinter with broad shoulders and a narrow waist.  According to Beverley Brook, Fleet is married to a fae and has got "like a gazillion foster kids". The children are adopted.
She has many dogs, and the captain of her dogs is a border collie called Ziggy.
She has, or appears, on a show on Radio 4 reviewing art exhibitions.