Nathan Price: he goes to the Congo to convert the people there, and to bend everyone and everything to his viewpoint, to his world perspective. It doesn't work. Take for example his attempts to plant beans. He plants them like he would back home, and ignores the advice given by the people there, to make mounds. His beans fail because he refuses to change. His refusal to change in the end undoes him, because Africa is a force that won't be changed. The more he tries, the more he fails. This eventually drives him mad; at the end, it is rumored that he was a wandering madman before dying.
Adah Price: Africa gives her the strength that she needs to overcome her weakness. In Africa, she is not deemed crippled; many of the children there are crippled. She is the only one that makes it out of there with her mother, walking on her two feet. Her symbolic healing occurs in Africa, with the literal healing later. Africa gives her the strength to make herself "whole".
Leah Price: She becomes completely a part of Africa's plight. She connects with the people there, becomes a part of the struggle for everyday food, and fights the battles that the people there fight. The Congo converts her to itself, keeping here there; it stays in her blood and wins her. She puts down roots and family in Africa, and continues its story through her children.
Rachel Price: She becomes symbolic of the white man that comes in and manipulates the circumstances of colonization for their own good. In the end, she doesn't leave, but she takes over a hotel and gleans profit from the white men that come to visit and control the nation. The Congo doesn't change her, it just gives her opportunities to fill a role that she is comfortable with.
Ruth Price: She is symbolic of the sacrifices that the Congo requires of anyone coming there. Her father comes to change the Congo, which is impossible; Ruth is the sacrifice of trying to do that.
Orleanna Price: She is forever scarred by what Africa asked of her. She went, she struggled, she was exhausted and drained, she tried to make the best of it. She was an unwitting victim in her husband's quest, and it took her heart and soul when Ruth died. She was never the same again; the Congo haunted her for the rest of her life.
Those are the differing impacts that the Congo had on the characters. I hope those help, and good luck!