Who are the main characters in the novel, and what are their roles in the overall plot of the novel?

Son of the Revolution is not a novel, but it certainly reads like one. It is a fascinating, searing, and, at times, deeply tragic firsthand account of the turmoil of the Cultural Revolution in China. As we will see, the characters in the book and how they develop reflect the bewildering, frightening pace of change during this tumultuous period of Chinese history.
The story is told through the eyes of Liang Heng. He has been brought up in the People's Republic of China, a country under the firm grip of a Communist regime led by Chairman Mao. Both his parents are fanatically devoted to the Communist cause, and Liang grows up in an environment steeped in the reigning ideology.
In their unthinking devotion to the cause, both of Liang's parents display varying degrees of naivety. This will, in due course, come to have serious consequences for their family. We first witness this during the so-called "Hundred Flowers" campaign which began in 1956. This was an attempt by the government to encourage intellectuals to speak out against government policies and make complaints about abuses in the Communist system. Many of the subsequent criticisms were incredibly harsh; they were certainly too harsh for the government, which quickly backtracked on the campaign and returned to its default position of systemic repression.
During the Hundred Flowers campaign, Liang's mother had taken the opportunity to utter mild public criticisms against a supervisor. Unfortunately for her, this was held against her after the campaign was suddenly brought to a halt. The subsequent backlash against so-called "Rightists" caught Liang's mother in a new, bloody wave of oppression, resulting in her being publicly humiliated and sent into the countryside for "re-education" through labor.
Liang's father, Liang Shan, is different. He remains fanatically loyal to the regime despite its ill-treatment of his wife. As far as he is concerned, the Party is always right, even if it means that he has to divorce his wife (which he does). Because of his cold-blooded sacrifice of his marriage, we can see Liang's father as a symbol of what is about to come: a wave of fanatical terror that will sweep the land, destroying families and leaving chaos and bloodshed in its wake. 
Yet, at this stage of his life, Liang bears no ill-will toward his father. In fact, the opposite is true. Instead, he feels great resentment toward his mother for speaking out during the Hundred Flowers campaign. As far as he is concerned, she is the one to blame for his growing isolation and ostracism at school.
The onset of the Cultural Revolution changes the whole dynamic of the characters' lives and their relation to each other. Liang joins the Red Guards, shock troops of the Cultural Revolution and fanatically devoted to helping Mao drive out those in the Party deemed insufficiently radical. The campaign, however, quickly gets out of hand, taking on a dangerous, hysterical complexion as Mao's acolytes launch a full-scale attack against all forms of authority, anything that smacks of the old ways.
Ironically, it is now Liang's father, the staunch Maoist, who is denounced. Because he has a brother in capitalist Taiwan, he is immediately made suspect in the eyes of the Red Guards, despite his professed loyalty to the regime. In order to remove the taint of suspicion from himself, Liang must completely condemn his father and join so-called "struggle sessions," in which Liang Shan is publicly humiliated and harangued by a baying mob of Red Guards. The respective roles of father and son have been reversed. The inter-generational conflict at the heart of the Cultural Revolution is now made starkly clear in microcosm.
Yet, there is a greater irony still. Although outwardly Liang appears more fanatical than his father, it is noticeable that he is the first to have serious doubts about the Revolution and its course. The inevitable collapse of the Cultural Revolution into factionalism and internecine warfare causes Liang to become thoroughly disillusioned. He witnesses firsthand the poverty, hunger, and disease wrought by the Communist regime on the countryside. The Revolution idealizes peasant life yet, in its ignorant fanaticism, it undermines and destroys it, leading to widespread human suffering.
As the story progresses, we gradually see Liang Shan start to take on a different role in the story. Despite his appalling experiences, he still retains a sense of self, a sense of who he really is beneath the hardline Communist exterior. As a condemned "Rightist" and intellectual, he is doubly damned. Despite this, he still finds that the peasants look to him for instruction in Party policy. He is still a teacher, still a figure of authority. Despite all that has happened to him, his inner spirit has not been totally crushed. 
Much the same could be said of Liang Heng. He understands the importance of education and learning and strives to achieve his academic goals. His father has, in his own little way, pointed toward the recovery of a sense of one's self, a sense of one's own inner drives. However, he is too old and too set in his ways to redevelop his true self in a meaningful direction. That role falls to his son, Liang Heng. Through his education, his skill at basketball, and his love for Judy, he finally sees a way out. In doing so, he remains true to himself and the inner core of his being, irrespective of the various trials and tribulations that he has endured.