From Longman Dictionary of Contemporary Englishalter egoal‧ter e‧go /ˌæltər ˈiːɡəʊ, ˌɔːl- $ ˌæltər ˈiːɡoʊ, ˌɒːl-/ noun [countable] 1 → somebody’s alter ego2 LIKE/SIMILARsomeone you trust and who has similar opinions, attitudes etc Mrs Reagan was widely regarded as the president’s alter ego.
Examples from the Corpus
alter ego• Equally Odette and her alter ego Odile attract the prince less through their enchanted beauty than through their artlessness.• Margara seems very much the alter ego of her creator.• He was not much closer to Belafonte, whose alter ego he played in the show.• During the past couple of years, Jerry had practically become his younger alter ego.Origin alter ego (1500-1600) Latin “other I” 