al ‧ ter e ‧ go /ˌæltər ˈiːɡəʊ, ˌɔːl- $ ˌæltər ˈiːɡoʊ, ˌɒːl-/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Language: Latin ; Origin: 'other I' ]
1 . sb’s alter ego an invented person that you use to represent part of your character that is very different from your usual one:
Gissing used his fictional alter ego to attack Victorian morals.
2 . someone you trust and who has similar opinions, attitudes etc:
Mrs Reagan was widely regarded as the President’s alter ego.