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Lennie 
Although Lennie is one of the important characters in 'Of Mice and Men',  he is quite different from most of the other characters in that he undergoes no significant changes, development, or growth throughout the story and remains exactly as the reader encounters him in the opening pages.

Simply put, he loves to pet soft things, is blindly devoted to George and their vision of the farm, and possesses incredible physical strength.

Nearly every scene in which Lennie appears confirms these and only these characteristics.

Although Steinbeck’s insistent repetition of these characteristics makes Lennie a rather flat character, Lennie’s simplicity is central to the story. Of Mice and Men is a very short work that manages to build up an extremely powerful impact. Since the tragedy depends upon the outcome seeming to be inevitable, the reader must know from the start that Lennie is doomed, and must be sympathetic to him. Steinbeck achieves these two feats by creating a character who earns the reader’s sympathy because of his utter helplessness in the face of the events that unfold.

Lennie is totally defenseless.

He cannot avoid the dangers presented by Curley, Curley’s wife, or the world at large. His innocence raises him to a standard of pure goodness that is not actually realistic. His enthusiasm for the vision of their future farm proves contagious as he convinces George, Candy, Crooks, and the reader that such a paradise might be possible. But he is a character whom Steinbeck sets up for disaster, a character whose innocence only seems to ensure his inevitable destruction.