Rat it is one of Venice's rogues and scoundrels. He keeps a watch post not far from Nox's house in the Backstreets of Outer City. He can provide Scarlett with some information on the Outer City, such as all rogues take names of animal species.
Rat can provide Scarlett with an introduction to another rogue, Crowe, if Scarlett will do him a small favor (the A Rat's Work quest.) Scarlett's reward for that effort in child slavery and abuse, and subsequent trust-building with the scoundrel Crowe, will be learning to sneak up on people and kick them in the groin.