That hint is talking about a loyal pet dog bandit girl, whom you can find all over the abandoned town. (Game mechanics wise, all bandit girls can turn into her, once you select the right option.)
The manor's mysterious door hasn't been implemented yet.
You need Fawn, Also push the Slum pass guard or trick him... Up to you, them a must. and a really long sequence of events to which I will explain with a magic called.