The assistant theory seems to be ruled out.
If she is an assistant why is she never with him on official trips when one might need an assistant? The only trip she has been on has been a cozy vacation to Disneyland where they shared a room.
She has been seen hanging out with him at MK, Epcot, AK, and HS as well as an evening at WL when Nate was out of character and at Marias bday party not sure what business is getting done. Also RSVLTs has to assign him handlers on company trips, the New Orleans debacle proved he needs to have someone with him while flying having an assistant would have avoided a missed flight in order to refill a prescription.