17
With this episode, we have completed Rose's backstory and it is pretty clear at this point that the season is going to end with a multi-episode fight against the demons. Honestly, there isn't anything groundbreaking in Rose's backstory. Nothing that hasn't been done in fiction before. However, I still think it is executed fairly well and they have given enough time to it to allow viewers to build at least a little bit of a bond with her old squad. I think compressing all that down into a single episode might have gotten the story beats across just fine, but would have lost some of the nicer character moments, like this scene, which just made "Empty Chairs at Empty Tables" start playing in my head.
I am looking forward to the coming battle with the demons. I think this episode showed that they aren't afraid of showing lots of blood when needed. So, it will be interesting to see the reaction of our Japanese high school students to the realities of war as it happens around them.
Edit: I personally prefer the 10th anniversary edition of the song, but it is admittedly less visually interesting.