Thanks for the chapter! I really like the mystery and slow world building like this.
From what we can know:
1. She probably a guard, that know something happened. It explains why she entered the room at a good timing
2. MC probably summoned because of a phenomenom or something. The room is not locked, close to surface but the elf is quite cautious of MC. The room was made for a quick access.
3. The elf was neutral before she assumes MC was hostile. I think its safe to say that multiple people got isekai'd, both good and bad people, they definitely brought some changes
it ends just like that ? also in last page her finger is black from the ink but its back to normal next case
somehow the only entrance isn't blocked, he could literally just walk out
She wrote using her left hand