In John Steinbeck's 'Of Mice and Men', the mentally disabled man is Lennie Small.
Lennie is one of two migrant ranch workers at the center of Steinbeck's 1937 novella. He is physically enormous and exceptionally strong, but he has an intellectual disability and depends heavily on his friend George Milton for guidance and protection. Lennie's dream is to live on their own small farm and tend rabbits.
His great strength contrasts with his limited understanding of consequences. He loves soft things, but he cannot control his power, accidentally killing animals and eventually causing a fatal confrontation. That tension drives the tragedy and helps explain why George feels responsible for him.
A common mix-up is to identify George as the disabled man because George is Lennie's caretaker and closest companion. Candy is an older ranch hand, while Curley is the ranch owner's aggressive son. Steinbeck based parts of the novella on his experiences working alongside migrant laborers, and its title comes from Robert Burns's poem 'To a Mouse'.