Theory:

The story "The Last Leaf" is written by O. Henry, and it was published in 1907. The story is set in New York City during the pneumonia pandemic.
 
The story "The Last Leaf" narrates the true friendship of Sue and Johnsy and the sacrifice of an old painter Mr Behrman. The central themes of the story are friendship, love, care and sacrifice.
 
The characters of the story are,
  • Sue
  • Johnsy
  • Doctor
  • Janitor
  • Behrman
'Last leaf", though not an active character, can be seen as an important aspect of the plot.