Theory for future books (spoilers through Book 7)

In book 3 when they’re dealing with the Iron Tangle Katia whitelists the entire Daughters including Hekla and Eva as being allowed in their personal space.

Now why does this matter? Eva and Hekla are both dead…right?

They have made clear throughout the series that there’s a such thing as undead characters, AND floor 15 is Sheol, their version of hell.

I think that somewhere in the next few floors Hekla, Eva, or both will return and the team will find out when they invade their personal space.