Theory:

“All right,” said Johnsy. “Finish your painting soon for I want  to see the last leaf fall. I’m tired of waiting. I have to die, so let me go away peacefully like one of those poor, tired leaves.”

“Try to sleep,” said Sue. “I have to paint an old miner. I will call Behrman up to be my model.”
 
Sue rushed down. Behrman lived on the ground floor.

He was a sixty-year-old painter. His lifelong dream was to paint a masterpiece but that had remained a dream. Sue poured out her worries to Behrman. She told him how Johnsy was convinced that she would die when the last leaf fell.

“Is she stupid?” asked Behrman. “How can she be so foolish?”

“She is running a high temperature," complained Sue. “She refuses to eat or drink and that worries me a lot.”
Explanation:
 
After hearing Sue's concern, Johnsy said, "All right". Then she asked Sue to finish the painting soon because she wanted to see the last leaf fall. She also stated that she was tired of waiting and that she wished to die soon. She compared her death with the ivy leaves. She wished to depart from the world in the same way the poor, exhausted ivy leaves did. It indicates that she was tired of her illness and desired to leave the world.
 
Later, Sue asked her friend Johnsy to try to sleep for some time. Then she said that she had to paint an old miner. As a result, she chose to use their next-door neighbour, Behrman, as a model for her painting. Sue headed downstairs to Behrman's apartment on the ground floor.
 
The narrator then introduces the old Behrman to his readers. Behrman was a sixty-year-old man. He was a painter whose lifelong ambition was to create a masterpiece. However, he was unable to fulfil his aim. For him, his goal remained a fantasy. Sue told him about Johnsy's health when they met. She might have told Behram about Johnsy's illness because they both lived in the same flat, and he was a senior painter. As a result, the old man and the girls might have a good relationship with each other. Later, she told him that Johnsy was sure that she would die when the last leaf fell off the ivy plant.
 
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Mr Behrman, an old painter

After hearing Sue's words, Behrman asked, "Is she stupid?" He made the statement because how can a person's existence be compared to the shedding of leaves. Furthermore, Behrman questioned how Johnsy could think such nonsense. According to him, Johnsy's behaviour seemed to be silly. On the other hand, Sue said that Johnsy was suffering from a high fever. Also, she complained that Johnsy was not eating or drinking anything. Her activities made Sue sad. As she was her close friend, Sue tried hard to make her active. But Johnsy didn't listen to her words.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
RushA situation in which you have to hurry or move somewhere quickly
2
MasterpieceA work of art such as a painting, film, or book that is made with great skill
3
TemperatureThe measured amount of heat in a place or in the body
Reference:
National Council of Educational Research and Training (2006). Moments. The Last Leaf– O Henry (pp. 44-48). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.