Mr. Behrman is a main character in "The Last Leaf" because he plays a significant role in the plot and development of the story. He is an artist who selflessly sacrifices himself by painting a leaf on the wall to give hope and motivation to a young woman named Johnsy, who is suffering from pneumonia. His actions inspire Johnsy to fight for her life and find the will to live. Ultimately, Mr. Behrman's selflessness and artistic talent make him a crucial character in the story.