alter ego vs friend

alter ego and friend both are nouns.

Word	Noun	Adjective	Verb	Adverb
alter ego	Yes	No	No	No
friend	Yes	No	No	No

As nouns, friend is a hypernym of alter ego; that is, friend is a word with a broader meaning than alter ego:

alter ego: a very close and trusted friend who seems almost a part of yourself
friend: a person you know well and regard with affection and trust

alter ego (noun)	friend (noun)
a very close and trusted friend who seems almost a part of yourself	a person you know well and regard with affection and trust
	a person with whom you are acquainted
	an associate who provides cooperation or assistance
	a person who backs a politician or a team etc.