friend vs alter ego

friend and alter ego both are nouns.

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

As nouns, alter ego is a hyponym of friend; that is, alter ego is a word with a more specific, narrower meaning than friend:

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

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