Rusty Key
The Rusty Key is used to enter The Sewers. It is obtained from Gunther in a cutscene after donating 60 items to the Museum.
Its existence is first referenced by Jas and Vincent in a cutscene in Cindersap Forest. This cutscene triggers on or after Spring 11 of year 1, by entering the Forest between 9am and 4pm on a Monday, Thursday, or Sunday.
Once obtained, it can be found in the Wallet in the The Player's Menu on the Skills tab.