Secret messages are accessible within the game by entering specific passwords:

Password = Message
•Minori Kumiko Kiyoharu = Actually, Erika is Minori! And Minori is a/the older sister!
•Grilled meal = We never want to work all night again. -Iwata and Yamagishi
•Mommy, the company is scary. = I'm honestly not a mama's boy! I'm just a bit spoilt, you see. -Udopyu
•Oomachi in love = I can't express these feelings of mine. I want to tell you, even if it's just a little bit. I love you... -T.O