the other I ?

al·ter ego noun

\ˌȯl-tər-ˈē-(ˌ)gō also -ˈe-(ˌ)gō\

 

Definition of ALTER EGO

alter ego

n.

1. Another side of oneself; a second self.

2. An intimate friend or a constant companion.

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[Latin : alter, other + ego, I, self.]