That's actually a bug with Ren'Py, not the game. I don't remember what the fix is, but IIRC some developers have found a way to get it to stop happening. While I don't see that in this game, I do in others. I wish I knew how to correct it.
I found a fix for it. The existing textbox was not easily readable, so I implemented a custom one. I noticed this happening after adding that and changing the font used in the gui file. I fixed it by deleting all the saves, including those in roaming and starting it over again. No more annoying avatar blinks.
Some of that might have helped (no idea), but deleting the saves and starting over doesn't actually fix the bug. It just removes it entirely. The bug is something that happens rarely (again, I don't know the details) that then gets included with your saves. So as you proceed through the game with a save that has stored that bug, the bug will persist for the rest of that playthrough. Deleting the saves does make it so you don't see the bug anymore, but that's because you've completely eliminated the thing carrying the persistence; you haven't actually corrected the bug itself. It's definitely a way around the bug, however.
The story is already well underway to a harem possibility. The primary reason the tag isn't here yet is that not all of the girls know about one another. Only the two sisters know about each other, and also the other two girls aren't actually in a relationship with the MC quite yet. In the current F95 definition for harem, you need at least three LIs who all know about and accept each other as additional loves of the MC. As soon as either Tomoe or Raveena are in an actual relationship (and not just at the flirting stage) and that's known and accepted by both sisters (and she knows about both of them), it can be tagged as harem.
I can confirm it bakes into the save when it occurs. I found it so annoying I opted to remove the avatars completely by editing them out of the characters.rpy leaving only the "define" lines calling out the character names. Better no avatars the constant flash of one. If anyone wants that and not comfortable editing their own characters.rpy, I could post the replacement.
I didn't see this problem in my save at the very end of Chapter 13. When you load the save, did you try rolling back a few screens and then clicking through from there? It's possible you're too far past the point of "safe" saving, causing the behavior you're seeing.
Sorry,,,, my bad... it turn out that I load from chapter 12,, and skip the chapter 13... I feal embarrassed right now ..
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