brother vs sidekick

brother and sidekick both are nouns.

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

As nouns, brother and sidekick are synonyms defined as:

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

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

brother (noun)	sidekick (noun)
a male with the same parents as someone else	a close friend who accompanies his buddies in their activities
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