A spear carrier is a minor actor in a play or, by extension, a person whose actions are of little significance.
[1] In the world of opera, the term is sometimes used literally: When a male chorus is required, as in Aida, for example, the onstage "army", armed with spears or swords, usually consists of several singers and as many who remain silent, filling out the group.
The modern meaning has its roots in classical Greek tragedy; as plays such as Antigone and Oedipus Rex concerned the tragic fate of nobles, several nondescript soldiers or guards were required to appear in the background, and the term was used to describe the guards who just escorted the main characters.
[2] The term has survived verbatim but evolved in meaning from its metaphorical use, and today carries the general meaning of satellite in Modern Greek (for example, a natural satellite/moon orbiting another heavenly body or an artificial satellite orbiting the earth).
A spear carrier is the anonymous character cut down by the hero as he advances to save the menaced heroine.