Erin’s first name never comes up again.

Ellie Kemper plays Erin. NBC

When she's introduced on five, episode 23 ("Michael Scott Paper Company"), Erin goes by Kelly until she tells temporary manager Charles Miner to use her middle name in order to cause less confusion in the office and avoid mix-ups with Kelly Kapoor.

If the gesture was done simply out of courtesy to her new boss, it seems strange that she continues to go by her middle name for the rest of her stay on the show, even though it is reasonable to assume that she would have preferred to go by her first name outside of the office.

Even her foster brother Reed calls her "Erin" when Andy meets him on season six, episode 19.