Often called 'Doc', they are about the closest thing to being a Marine a person can be without actually being one.
Source: Sgt, USMC
Marine 2: He's inside with the rest of the squad clearing the building
Marine Sgt: Nice.
Marine 2: Oh here he comes
Corpsman: Buildings clear
Marine Sgt: Not bad for a squid
Marine Two: Doc had to take staff sgt to get his bore punched
A sailor that has the duty of providing medical support to the Navy as well as the Marines.
Blue Side Corpsman - works in hospitals, sick bays, ships, and on base with the Navy
Green Side Corpsman - trained in combatives and works in the field with the Marines. If respected and trusted his Marines will call him DOC.
GS "DOC, Im hit! RIGHT LEG RIGHT LEG!!!"