This is Serenity, Celestia's (and then Luna's) second in command in my story A Dubious Welcome. A relevant excerpt:

There was a muffled, "Oh!" and the pale green mare poked her head out from behind a pile of paperwork upon her desk, her glasses slightly askew and her cyan mane tied back in a messy bun held together with two quills.

"Morning, Princess! One moment, I was just working on some last minute adjustments to today's schedule."

Her horn glowed turquoise as she shuffled through one pile, quickly selected one parchment, tossed the rest onto a completely different pile, then trotted past Luna and down the hall, evidently expecting her to follow.