second fiddle vs foot soldier

second fiddle and foot soldier both are nouns.

Word	Noun	Adjective	Verb	Adverb
second fiddle	Yes	No	No	No
foot soldier	Yes	No	No	No

As nouns, foot soldier is a hypernym of second fiddle; that is, foot soldier is a word with a broader meaning than second fiddle:

second fiddle: someone who serves in a subordinate capacity or plays a secondary role
foot soldier: an assistant subject to the authority or control of another

Other hypernyms of second fiddle include subordinate, subsidiary, underling.

second fiddle (noun)	foot soldier (noun)
a secondary role or function	fights on foot with small arms
someone who serves in a subordinate capacity or plays a secondary role	an assistant subject to the authority or control of another