The character in the image below appears a few times throughout the series. She looks a bit like Yamada but Yamada is never seen wearing glasses while talking to the rest of the staff.
The image was taken at the 12:10 mark of EP11 but she appears in other episodes as well, although rarely.
that is Maya Matsumoto, the self proclaimed "perfectly normal" member of the staff.
On the 13th episode of the first 2 Seasons she has a speaking role in which in the first season she actually introduces herself. she also has a speaking role in the 3rd episode of Season 3
Also in the Artbook that comes with the Season 2 Premium Edition from NISA you can find her making comments on a number of pages
