In spite of his crime, Raskolnikov is worth redeeming and therefore since Raskolnikov is a dual personality, Dostoevsky also felt the need of creating two additional characters who, taken separately, represent the two opposing aspects of his nature. Thus, Sonya is the warm human, compassionate, charitable aspect of Raskolnikov's character. She is the meek and submissive personality. She will become the symbol of "the suffering of all humanity."
In contrast Svidrigailov is the detached cold manifestation of the self-will and power and intellect. Consequently, so often in the novel, when Raskolnikov is attracted to Sonya, he is repulsed by the depravity of Svidrigailov. Likewise, when he is talking or going to see Svidrigailov, he is disgusted with and repulsed by Sonya's tears and weaknesses.
With Raskolnikov's character established as a dual one, and with these two characters, Sonya and Svidrigailov, representing the two alternating aspects, the general pattern of the novel is to bring Raskolnikov back into one functioning character. Thus, we must have two redemptive characters. Here the importance of Sonya's role is seen. As she represents one aspect of Raskolnikov's personality, so must she function as the person who is to redeem that aspect. Therefore, Sonya is the redemptive figure for Raskolnikov's humane aspect of his personality. Through her suffering, she makes him realize the importance of a love for humanity, that a human being cannot be "a louse" or a parasite sucking the life from other human beings. But it should be emphasized that Sonya does not accomplish her purpose by overt action. She is the passive figure whose simple presence is enough to inspire Raskolnikov's actions.
The other redemptive figure is Porfiry. Here is an intellectual man who uses his intellect for the good of man. He sees in Raskolnikov the potential of a great being who made up a theory and then was ashamed that it broke down. Porfiry is the man who recognizes that the theory is base but that Raskolnikov is far from being base. His purpose, therefore, is to make Raskolnikov see the difference between the baseness of the theory and the ever-present potential in himself. What Porfiry realizes is that any idea, if it is conceived of for the benefit of humanity, must be a human idea and must be executed by a humane person. He tries to make Raskolnikov see that the idea is base because it views a large portion of mankind as base.
Thus Raskolnikov at the beginning of the novel is a dual personality with two aspects of his personality represented by Sonya and Svidrigailov. The problem is to bring this personality into an integrated whole. This task is assigned to Sonya and Porfiry. The emphasis is that man cannot separate the humane aspect of his life from the intellectual aspect. Whatever man does must be done in terms of the betterment of general humanity.
Raskolnikov's punishment, that is the general suffering he undergoes, is a result of this split personality. It was one aspect that murdered the old pawnbroker, but it is the humane side that must suffer for the murder.