Final answer:
Mirror images and recurring motifs in literature often symbolize a character's obsession or singular focus, as seen in the thematic depth of Edgar Allan Poe's 'Usher.'
Step-by-step explanation:
The symbol used to suggest the narrator's singular focus on Mangan's sister is not explicitly stated in the excerpts provided. Generally, in literature, symbols such as mirrors or recurring images may be used to reflect a character's obsession or focus. For instance, in Edgar Allan Poe's story, mirror images symbolize the reflection of doom and intensify the theme of hopelessness in 'Usher.' Similarly, other works might employ specific objects, colors, or motifs to symbolize a character's fixation on another character or idea.