What's Lennie's role in Of Mice & Men?

I think a better way to look at this is George and Lennie together. Both men function almost as one. They are the protagonists in the novel. Steinbeck uses them to explore not only their relationship but also the world they were trying to navigate through. George and lennie are very much an anomaly in this world of itinerant ranch hands. On a superficial level, one is clever and smaller while the other is "dumb" and big. Dig a little and you find a friendship based on unconditional love and loyalty. We examine this against an aching loneliness that permeates ranch life. We really want them to have this dream but their understanding of each other does not extend to their ranch society. In the end George and Lennie are alone which makes the ending that much more tragic.