their middle child was a mole.
That essentially turned out to be the case-- or a rabbit, who liked to burrow and created a cozy little nest down there.

This reminds me of the Kinsey Milhone books ("A is for Alibi"), where her landlord winds up reworking a garage to make a fully self-sufficient studio with a loft bed and a stacked washer/dryer set. The character loves that small, perfect space, and your character did, too. It was a happy place for him/her, and helped keep a nice balance of being close to his/her family but never being overwhelmed. I think a lot of introverts can relate to that!