Spirit Hunter: Death Mark II – True Ending Guide

To start the true ending, you first have to clear the game with the good ending.

Then, in the main menu, select Continue → Chapter selection, and select the save that has the good ending cleared.
The save doesn’t seem to have to be a save of “Chapter 7 The End” save, just any save that you’ve previously had “Chapter 7 The End” saved on and used Chapter selection on, so you should still be fine even if you saved over it.

Select the save and then load Chapter 6. When prompted to change your allies’ fates, make sure all your allies are alive, if they aren’t already.
(You may need to clear the game with all your allies surviving first though, haven’t tried otherwise myself.)

When encountering Sakamoto in the faculty room, answer their question with the response about Douryou and Kinukawa.
If you loaded from a good ending save, this should cause a new prompt to appear later in the conversation.
For that prompt, choose the option to convince her.

Play through the chapter normally until you reach the Suspensive Act in the clock tower.
In this scene, choose the options to threaten with the wrench and defend with the crowbar. Doing otherwise for either of these choices will lock you into the good ending instead.

Proceed with Chapter 7 in the same way as the good ending.
If you followed the guide correctly, you will not receive Sakamoto’s report and she will not appear in the forest.

Additionally, you should get a new prompt and event after exiting the clock tower for the second time during this chapter. If you choose to enter the clock tower, you’ll trigger the true ending.

Clearing the true ending also unlocks the character profile for The Departed.
Also, thanks to Steam user “yoyo” for helping me find the true ending in the first place.

If you only follow Key Point #1 under the Chapter 6 portion of this guide and ignore the rest of the contents of this guide, you can unlock an additional scene in the good ending instead. You can’t see the new good ending contents in the true ending, so you may want to make multiple saves before entering the clock tower for a second time in Chapter 7 if you want to see both.