second banana vs underling

second banana and underling both are nouns.

Word	Noun	Adjective	Verb	Adverb
second banana	Yes	No	No	No
underling	Yes	No	No	No

As nouns, underling is a hypernym of second banana; that is, underling is a word with a broader meaning than second banana:

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

Other hypernyms of second banana include foot soldier, subordinate, subsidiary.

second banana (noun)	underling (noun)
a performer who acts as stooge to a comedian	an assistant subject to the authority or control of another
someone who serves in a subordinate capacity or plays a secondary role