Short answer: you address the agent. She is your intended reader.When an agent requests on their website that all email queries be sent to their assistant, do you then open your letter by greeting the agent...or the assistant? Or both?
Much like you start your letter to Her Majesty the Queen with "Hey Your Majesty" even though her long-suffering Lady in Waiting, Priscilla of the Pen and Papyrus, will be reading it first.
You may wonder why agents have queries sent to their long suffering Assistants.
Mostly it's to weed out the ones that don't need the agent's attention.
The ones where someone hasn't finished their novel and just needs a "quick assessment of whether they're on the right track."
The ones where the comps are Mickey Spillane and Harold Robbins.
The ones who request a reply this week, or they'll just self-publish.
And prioritize the ones that need immediate attention.
(copy my assistant Felix Buttonweezer of course)