Describe the role of sue in …

Sue told Dr. Behrman about Johnsy's superstition and the reason for her ill will, which helped Dr. Behrman find a solution to Johnsy's problem and helped her recover. Sue gave Johnsy the positive outlook and motivation to survive, she did not leave Johnsy alone on her darkest days and thus, became instrumental in saving her.
 

Sue is a true friend who dearly loves Johnsy. She tries to motivate Johnsy by humming and being cheerful as she sits by her friend drawing. But, when she realizes the Johnsy has counted the leaves that have fallen from a vine, she "looks solicitously out of the window." Calling her friend "dear," Sue asks Johnsy what she counts. When Johnsy tells her that with the fall of the last leaf from the vine she must go, too. Sue acts scornful of "such nonsense"; speaking positively, she tells Johnsy that her thoughts about death are "naughty." For, says Sue, who lies in order to convince Johnsy, the doctor has said that her chances of getting well were "ten to one."

Sue pretends that the situation with Johnsy is not of the magnitude that it is; in her love and hope, she acts as though Johnsy will soon be well. However, she is truly worried. So, she begs Johnsy to close her eyes because she needs the light in order to continue her drawings. Johnsy agrees, but she tells Sue to hurry as she wants to go down like the last leaf. she goes to Mr. Behrman, informing him of the gravity of Johnsy's condition, hoping there is something he can do. Angered that Johnsy is ill, Behrman complains, then, comes to pose, Sue takes him outside to look at the barren vine with one last leaf.

So greatly concerned is Sue about her friend that she effects the change in Johnsy's psyche that causes her to become well. Sue's determination to do what she can for her friend saves Johnsy's life. There is no doubt that Sue loves Johnsy as she has thought about what the doctor has told her and done everything she can to save Johnsy.