An archetype is a model on which others are based. Examples of archetypes in literature would be "the mad scientist" (Dr. Jekyll, for example); the strong, invicible hero (think Beowulf), etc.
Archetypes in this story by O'Connor include Hulga, her mother and Manley Pointer.
Hulga is an archetype because she is a character who is a young woman who is trying to rise above what she feels is a humble Southern upbringing. She thinks she is better than others and smarter than others. This is evident in her rude comments and her attitude towards her mother and Manley Pointer. In the end, she gets her "comeuppance" and is humbled by Manley Pointer, who has tricked her and completely shattered her former view of herself and of others in the South. eNotes states:
All of a sudden, her intellectual snobbery in her nihilism becomes reduced to the same as his manipulative cruelty. After he abandons her, humiliated, in the barn, she sees “his blue figure struggling successfully over the green speckled lake.”
Hulga's mother is a stereotypical Southern mother who has tried to raise her children in the best way possible and who tends to dote on them. She is down-to-earth and in touch with her roots and does not try to act or be better than others as her daughter Hulga does.
Manley Pointer is, in many ways, an archetypal "bad boy", although we do not know this until the end of the story when he steals Hulga's artificial leg and leaves her helpless in the loft. Manley preaches Southern religious beliefs and sterotypes, but in reality, he is a farce, a cad, a fraud.