To unlock Digby you will need to purchase a map from the witch's shop on every main floor. (floors 1-4 Ground Floor, Upper story, Attic Sanctum, Laboratory)
Each map will cost $200, so you will need to save some money and find the witch's shop each time. If somehow you end up missing a floor you can buy a map when you are on a looped run (having jumped off the balcony and starting over back on the ground floor)