Definition of "alter ego"

[…] a belief in the possibility of a man possessing an alter ego in the form of some animal such as a crocodile or a hippopotamus. It is believed that such a person's life is bound up with that of the animal to such an extent that, whatever affects the one produces a corresponding impression upon the other, and that if one dies the other must speedily do so too.

1897, Mary H. Kingsley, Travels in West Africa, page 538

As one commentator has remarked, all the confusing old three-syllable names that used to index Chinese politics can now be forgotten — the names and their bearers have ceased to be of the slightest importance. In their place stands one man — a small, frail, volatile but tremendously energetic man — who oddly enough has become the alter ego of that powerful, sprawling, sleepy giant that is China.

1938, Robert Berkov, “China's Chiang”, in Strong Man of China: The Story of Chiang Kai-shek‎, Boston: Houghton Mifflin Company, page 1

Ruffalo's turn as Bruce Banner and his angry green alter-ego has been widely praised. Avengers Assemble (titled The Avengers outside the UK) is on course to be one of the year's biggest films and has received strong reviews.

2012 May 1, Ben Child, “Mark Ruffalo gets green light to play Hulk in six-movie deal”, in the Guardian‎

GWM, 5'10", 175, 40, non-smoker, MCC-er. Good build. Int. in classical music, books, travel. Seeks alter ego with own peace for honest relationship.

1974 April 13, “Sex Isn't Everything! (personal advertisement)”, in Gay Community News, page 10