The PillowCase is embroidered with a faded kanji character— "Nostalgia" (懐). Clicking on the pillowcase during the free-roam segments (6:00 AM - 7:00 AM) triggers flashback sequences revealing why the siblings were separated. These flashbacks are only available in v1.0. Patch v1.1 moved them to a separate "Memories" menu, breaking immersion.
Here’s a sample interesting review (as if from a player): 30 Days - Life with My Sister -v1.0- -PillowCase-