Narrative voice (in memoir or fiction) is another word for the self or person, the guide who is relating a human story to a reader (or listener). Reflecting a specific real or invented personality, the narrative voice embodies a history, a world view, a capacity for empathy, love, and joy and at the same time a complex set of desires, fears, and neuroses. Whether authentic as in memoir or believable as in successful fiction, the voice/personality is the reader’s core connection to the storyteller's unique understanding and version of the story he or she is telling. The voice/person telling the story is the key to plot, theme, and reader engagement in literary narrative.
In memoir, of course, the voice telling the story is you.