In the story of David who is Goliath?

Goliath and Saul

The underlying purpose of the story of Goliath is to show that Saul is not fit to be king (and that David is). Saul was chosen to lead the Israelites against their enemies, but when faced with Goliath he refuses to do so; Saul is a very tall man, but Goliath is a giant.

Goliath is described in the biblical Book of Samuel as a Philistine giant defeated by the young David in single combat.