I don't mean to start any trouble, but I suppose it could be oversight/mistake on the author's part. She started as Marcius' messenger and was then "retconned" (without an actual retcon on the initial 3 chapters) to be the princeps messenger.There are apparently contradictory statements in game about who Zephyr reports to, which certainly leads to some confusion.
In ep1-3 we only encounter the Zephyr running messages for Marcius. In ep1 Andrew refers to her as "The archon's zephir".
But in ep3 when the Zephyr is delivering Marcius' invitation to meet, she states: "I am a messenger under the auspices of the princeps."
In ep4-5 the Zephyr is running messages for the Princeps, and she apparently has direct access to his quarters. So I believe the Zephyr has always been in service to the Princeps.
It's not entirely clear why she was running messages for Marcius, but not other Archons. Initially it was probably because at that stage other powerful people weren't aware of MC/Sharon, Marcius is the only one who wants to send messages to them. Alternatively it may be because Marcius was the most powerful Archon in the city so had more privileges.
If it's the former, and all Arcohn's have access to the Zephyr's services, then Sharon should too. Perhaps she does, but simply doesn't know. Sharon doesn't seem to know much about how the bosses run/do things.
Thing is that, assuming she's trustworthy in regards to how discreet she is (which shouldn't be assumed by anyone who isn't naive), she'd be a great help in some time saving scenarios in which characters go back and forth from the main city to Scottstown. But as of now she has been presented as a "tool" that others have and can use, but not the MC/Sharon.