Lennie is an interesting character in Of Mice and Men in that he simultaneously exemplifies both power and powerlessness, two central themes in the novella.
Lennie has the intellectual capacity of a child. He has a limited understanding of consequences and difficulty controlling his impulses. He (usually) can tell the difference between right and wrong but is often unable to stop himself from acting on his urges. Due to his mental incapacitation, Lennie is completely reliant on George. He fully depends on George to guide him and keep him safe. In this sense, Lennie is powerless.
Lennie’s only power lies in his physique. Although he is mentally comparable to a child, he is physically very large and strong. He comes off as a threatening presence to those who do not know him well and do not know his kind, gentle disposition.
Lennie demonstrates an internal struggle between power and powerlessness. His physical power is eclipsed by his intellectual powerlessness. His mental limitations turn his physical power into a liability by preventing him from realizing his own strength and understanding the consequences of his actions.