Do you mean the last one on the ending list??? Or just that you're missing one ending? I got the last ending. (Spoilers for how to get it below)
Remember the scene where the thugs rob the store and you can choose one (non-Labibap) collectible to take with you? Take a tearful boy and then play as if you're going for the "happiness ending" (basically don't die before you can make the choice you need to make). Eventually you'll get to a dialogue option you haven't seen yet - you should recognize it when you get to it.
(ALSO I love your games, First Kiss at a Spooky Soiree was one of the first LGBTQ games I played, right around when I was first realizing that I was bi, so it holds a special spot in my heart, haha)