Visit Kokkol after you've progressed through the Memoir Pages side quest as far as possible (collect all 20, bring them to Branch Castle library, then visit Kokkol).
You should receive the key for the locked house in Hillsdale if you talk to Nero with Ami as the party leader (in the game's second part only). Nero's home (in the game's second part) can be found on an island on the central northern edge (northwest of Palumpu).