The SEAs from each branch at the 2009 Commander in Chief's Ball
A Senior Enlisted Advisor (SEA) in the United States Armed Forces is the most senior enlisted service member in a unit, and acts as an advisor to the commanding officer. Formal, E-9 billets for the senior enlisted advisor are established at Service unit (e.g., battalion, wing or higher), command, major command, force or fleet levels to the SEAs/CSELs of DoD Agencies and the Senior Enlisted Advisor to the Chairman of the Joint Chiefs of Staff. SEAs are also known as Command Senior Enlisted Leaders (CSEL). Always a non-commissioned officer, the SEA is the main link between the commanding officer and the enlisted service members under his or her charge.
In the United States Army, the SEA for a battalion or larger-sized unit is usually a Command Sergeant Major. For Marine battalions and larger, Sergeant Major typically takes the role. Navy ships and other commands will usually have a Command, Fleet, or Force Master Chief Petty Officer (known as the Chief of the Boat on a submarine), while an Air Force Command Chief Master Sergeant serves at Wing, Numbered Air Force, Field Operating Agency (FOA), Major Command or at "Joint," DoD Agency levels.
International equivalents 
In Australia the equivalent positions within the Royal Australian Navy (RAN), Australian Army and Royal Australian Air Force (RAAF) are Warrant Officer of the Navy (WO-N), Regimental Sergeant Major-Army (RSM-A) and Warrant Officer of the Air Force (WOFF-AF) respectively.