Candy is an old, disabled worker on the ranch whose only ally is a dog he has owned since it was a puppy. Later on in the novel this dog is declared 'past it' and shot for being old. Candy is then left on his own and compares himself to his now deceased dog saying that when he's declared too old to work no-one will shoot him, he'll just get given a final payment which won't last very long and then get 'canned'.
Slim is also a ranch worker who has been travelling in search for a job. He is a trusted person that George confesses the previous acts of Lennie to, he seems to understand the friendship between Lennie and George, with him most likely wanting a companion like they have in each other. The first quote also applies to him.
Crooks is a black veterinarian, the only black person who works on the ranch. He symbolises all the black people of the time period-solitary because of racism towards them. Crooks is also lonely by his own doing in a way; he is a proud man who distances himself from the other workers. He sleeps in a room just off the stables with the horses which could be inferred that the boss and other workers consider him an animal which separates him from everyone. But his attitude towards the other workers could suggest that he chose to separate himself from them anyway, not wanting to be close to the racist's that discriminate against him.
Curley's Wife is probably one of the loneliest characters in the novel with her name giving away most of the reason why. The fact that she has no name of her own and is constantly referred to as Curley's wife shows that she is seen as a possession of her husband, and her husband contributes to her loneliness by attempting to distance her from everyone else on the ranch, seeing her as his, and his alone. Curley's wife in this novel represents all women of the time period, with the loss of their own identity in favour of their new husbands. The women during the great depression were seen as a prize that men got and they were expected to keep quiet whilst being the perfect housewife (cooking, cleaning and making babies). Taking this into consideration it could be inferred that Curley is also lonely by the fact that he wants to keep his wife to himself. This could be his misguided way of having a permanent companion in his wife.