The Narrator has one telltale feature: a birthmark on his foot. As a younger man, the Narrator learns that the birthmark greatly resembles (but is not) an early-stage tumor, which could have engulfed his entire body, killing him. The birthmark symbolizes the constant presence of death in the Narrator’s life: the birthmark is a reminder that the Narrator could be dying of cancer right now. The Narrator’s most unique feature—the one that allows strangers to identify him to a certainty, and which thus plays a convenient role in the novel’s plot—is his birthmark, an essentially meaningless part of his body, over which he exercises no control. Thus, the birthmark could be said to symbolize the Narrator’s uniqueness (as opposed to Tyler’s kiss, which is shared by of Tyler’s followers), but also the flimsiness of the very concept of individuality.
The timeline below shows where the symbol Birthmark appears in Fight Club. The colored dots and icons indicate which themes are associated with that appearance.