It goes without saying that the following involves spoilers, so make sure to come back when you're ready!

In Chapter 11, you're given the option of inputting a password. By typing in Dreizehn , you will get the normal ending. But afterwards, you'll learn the existence of a special, true ending!

The secret password is "Dreizehn Ardelyte"

This revelation reveals that the Doctor, Belphomet Ardelyte, truly considered each of his creations as a member of his own family, allowing Eumenides and Dreizehn to unleash their full power to put an end to the mechanical dystopia that is Aiolon. And following from that, we learn how Dreizehn met the Tanuki that guides her into the Festival of Wolves...