Shield bearer usually refers to a Macedonian version of a heavy armored

hoplite Hoplites ( ) ( ) were citizen-soldiers of Ancient Greek city-states who were primarily armed with spears and shields. Hoplite soldiers used the phalanx formation to be effective in war with fewer soldiers. The formation discouraged the sold ...

, whose tasks were protecting flanks of the sarrisa phalanx, and carrying a shield to protect other people in same ranks.Rodan, Simona.
Aegean Mercenaries in Light of the Bible: Clash of cultures in the story of David and Goliath
'. Archaeopress Publishing Ltd, 2015. 20. A commander might be protected by several shield bearers. In combat, the shield bearers served as core of Macedonian army along with sarissa pike men, usually protecting flanks of the pike infantry. Other types of soldiers that made use of shield bearers included charioteers, archers,

crossbow A crossbow is a ranged weapon using an Elasticity (physics), elastic launching device consisting of a Bow and arrow, bow-like assembly called a ''prod'', mounted horizontally on a main frame called a ''tiller'', which is hand-held in a similar f ...

men, and early handgunners.

References

Military units and formations of antiquity Ancient Greek infantry types {{Mil-hist-stub