The character of Lennie in Of Mice and Men is disadvantaged in several significant ways.
The most obvious disadvantage Lennie suffers is his lack of mental and emotional development. In the opening chapter, Lennie is depicted as childlike and "slow". Keeping a mouse in his pocket and accidentally killing it, we see that Lennie is, in a way, innocent and unable to predict the consequences of his actions.
Lennie must be prodded also to remember simple things and even forgets the most important episodes of his past, both the recent past and the most distant past. A final detail regarding his mental development: Lennie cannot easily understand social situations the way normal people do. We see this when he visits Crooks, when Curley attacks him, and when Curley's wife comes into the barn at the end of the story.
Another disadvantage for Lennie is a lack of family. The woman who raised him, his Aunt Clara, is dead and Lennie has no family remaining. George is his only friend.
Finally, Lennie has no material wealth. He shares this disadvantage with the rest of the workers on the ranch who are forced by necessity to do hard labor for pay and who have "nothing to show for it" in the end.
Lennie is disadvantaged intellectually in the novel. He is physically strong and powerful but he doesn't have analytical ability, and he doesn't know his own strength. His inability to remember information is the root cause of many of his conflicts in the novel. George his friend and caretaker of sorts is his Foil in the novel. All of George's strengths illustrate a weakness of Lennie's.