sidekick vs brother

sidekick and brother both are nouns.

Word	Noun	Adjective	Verb	Adverb
sidekick	Yes	No	No	No
brother	Yes	No	No	No

As nouns, sidekick and brother are synonyms defined as:

sidekick and brother: a close friend who accompanies his buddies in their activities

Other synonyms of sidekick include buddy, chum, crony, pal.

sidekick (noun)	brother (noun)
a close friend who accompanies his buddies in their activities	a male with the same parents as someone else
	a male person who is a fellow member (of a fraternity or religion or other group)
	a close friend who accompanies his buddies in their activities
	used as a term of address for those male persons engaged in the same movement