Looking back, I'm not entirely sure the girl that the spider killed isn't actually just the spider itself. From her dreams it looked like she was already transforming into a spider and his reactions in her "dreams" matched up to his reactions in real life. Maybe that death and everything was just her inner psyche as she turned into the spider-monster, losing her more human consciousness. That would explain why the everything, including the brain, was missing. In fact the going back it looks like the girls body was blown open from the inside out. Perhaps all that's left is her bare desire to be with Rei when she switched bodies?
If so it would make sense if there could be a "second spider" for the series, which this lady looks like she's going to become.... or a snake. That'd be more fitting to her personality.