Here's a simple guide to get the Harem ending because it kind of involves using a log that you don't have on hand. In other words, you have to sort of cheat to get this ending.
Go through the reactor meltdown with *Mute, then continue down her path. Close to the end, she will ask you whether you've shown her everything. Tell her there's one more thing left.
Now, the tricksy part. If you've gone through Endings 1 and 2, you'll know about that last diary entry of the Pale Bride. You know the one. Yes, that one. What you want to do is show it to *Mute. Ordinarily, you can't, because you need to go down the entirety of *Hyun-ae's path to access that entry and now, she's locked out.
This is where you learn how to cheat:
Notice that you're able to search logs by ID. This allows to search for every log in the game, including those that you couldn't unlock.
So, with this in mind, use the ID for that diary entry to unlock it:
Then, show the log to *Mute. Yay!