I would argue that Cliff is presented as the antidote to Jimmy. While Jimmy is angry, Cliff is calm. While Jimmy is prone to throwing around verbal abuse, Cliff is a peacemaker. Cliff is warm, humorous and has a loving nature. This creates a stark contrast with Jimmy's coldness, anger, and dissatisfaction with life. Cliff has a working-class background, as opposed to Jimmy, who has a better education behind him.
Cliff's calm temperament enables him to take the constant jibes thrown his way by his "friend" Jimmy without becoming upset or taking them personally. I would also describe Cliff as a hopeful character who, unlike Jimmy, does not allow his thoughts to be controlled by angst and distress about the state of modern England.
Cliff is presented as a good friend, both to Jimmy and Alison. His tolerance of Jimmy's ongoing displays of rage and his willingness to work with Jimmy in his candy stall business show that he is Jimmy's friend. His ongoing efforts to protect Alison from her husband's sharp tongue and callous ways show that he is, without doubt, a fantastic friend to Alison.
Despite the fact that at various points, one gets the impression that Cliff has feelings for Alison, he never acts on these feelings or behaves inappropriately. Cliff is presented, from start to finish, as a gentleman.