How does Lennie's character progress through the story?

Lennie's character actually remains the same even if his living circumstances change, and this static quality is at the heart of the tragedy that sets Steinbeck's novella apart from other works of literature that concern the death of a main character.
Lennie's innocence is undeniable, as his child-like mindset never matures and consequently, he never learns from previous conflicts or problems. Lennie is a danger to others and to himself, because he has the strength of a large able-bodied man but the desires, the intellect, and the awareness of a small child. He is unable to gain insight into himself and he reacts to difficult situations with extreme emotion, like a youngster with no ability to regulate himself. Unfortunately for George and for Lennie's innocent victims, Lennie's inability to change is the ultimate cause of Lennie's downfall, administered tragically by his best and most loyal friend.

Lennie's character does not progress throughout the story. Lennie is a static character, which is a character that does not change throughout a story. At the beginning of the novella, Lennie is portrayed as a mentally handicapped, dependent individual, who mimics George and relies upon George's friendship to help him through life. Lennie also has an affinity for petting soft things, which is shown in the opening scene of the novella when he continually strokes a dead mouse. George also elaborates on Lennie's incident in Weed, which almost got them killed.
As the story progresses, Lennie's character remains the same. His lack of intelligence and inability to comprehend various social situations is evident throughout the entire story. Lennie also remains loyal to George and relies on his guidance to the end. Lennie's static character is also portrayed in his obsession with petting his puppy and stroking Curley's wife's hair. Lennie's reaction to Curley's wife also mirrors the incident in Weed. Overall, Lennie remains a dependent, helpless, unintelligent character throughout the entire novella and does not change.