There are actually two instances of it. The easiest one to find is by completing the maze game at the back left of the mansion. That will open the door with the picture of a tiger girl done by bubbles pup.
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Would it require you to find the common key to open that door? Unless it is hidden somewhere obvious and I'm just incredibly blind, but I don't exactly know where the common key is hidden.
Sorry if I am asking a lot of questions, but I am not sure if I am missing something obvious during my playthrough