Having defeated the anogratch army in the previous book, the protagonist now has the opportunity to evolve into either a taratect or a small poison taratect. Naturally, she picks the latter–it's a rare species, after all! That plus her newly acquired Pride skill grant her a big powerup, and she enters the Middle Stratum of the Great Elroe Labyrinth.

It's lava everywhere! Her thread, which carried her through the last book, is now useless, igniting shortly after she produces it from the immense heat. Somehow, she has to get through a region filled with powerful monsters without her most effective skills. How will she do it? Happily, she can now develop her magic.

This volume corresponds to Chapter 15-1 through Chapter 23-1 of the manga.

The protagonist's identity

In this volume we see Kengo Natsume and Yuika Hasebe. The former was already mentioned in a flashback (and is male, anyway), but the latter is new information.

We are additionally told that Temarikawa and Furuta were "relatively quiet girls" who were friends with Hasebe. The protagonist describes herself as a friendless gamer, so these two are probably out of the running.

Additionally, in the future chapters, we're told that the demon lord became active shortly after they were all reborn, and they say a bunch more suggestive stuff:

• “Ah, sure, sure. The Nightmare of the Labyrinth… That was the spider monster that dealt Julius his first and only defeat, right?” / “I overheard him talking about it with Shun. It sounds almost like the demon lord!”
• “Hmm. Say, this is just a thought, but…a new demon lord took over right around when we were born, right? Could you imagine if the demon lord was one of us reincarnations?”
• “I doubt a baby could become a demon lord so quickly unless it spent every waking moment from the time it was born moving around and leveling up.”

Clearly, we're meant to infer that the demon lord is the future version of the protagonist. But it's so heavy-handed that it seems likely to be a red herring, doesn't it?