In literature the terms protagonist and antagonist are used to refer to, respectively, the main character of the text and the character who seeks to oppose the main character in achieving what he wants to achieve or doing what he wants to do. Given this definition, it becomes clear that the protagonist is quite obviously Frederick Douglass himself as he tells his story. The text therefore features many antagonists, which mainly take the form of the white owners that Douglass has to contend with and survive. Interestingly, one of the first antagonists, Mr. and Mrs. Auld, is actually also the character who ironically allows him to develop his aim in life, which is that of freedom. When Mr. Auld prevents his wife from teaching Douglass to read any further, Douglass understood the secret behind the white's enslavement of blacks:
I now understood what had been to me a most perplexing difficulty--to wit, the white man's power to enslave the black man. It was a grand achievement, and I prized it highly. From that moment, I understood the pathway from slavery to freedom.
However, even though at this moment Douglass clearly has something of an epiphany about his own condition and what he wanted out of life, he still had to face many difficulties in the form of the physical abuse and other forms of abuse that his various owners forced upon him. They are the antagonists in this text as they sought to prevent Douglass from gaining the freedom that he so desired.
In the novel, Frederick Douglass is of course the protagonist. An argument that Mr. Covey is his primary antagonist can be made. While all the slaveholders and the overseers are represented as corrupt, cruel, and evil, Douglass manages to deal with all of them in his own way. Covey is another matter, however. Douglass has not lead a charmed life by any stretch of the imagination, but it is when he is sent to stay with Mr. Covey that he truly fears for his life.
Covey is a notorious "slave-breaker." Especially "troublesome" slaves are sent to him for a year, and within that year he does everything he can to break the slaves' spirits. He is referred to as a snake by the slaves because of his hiding in the grass in order to catch the slaves taking a break from their labor. He puts Douglass in charge of some oxen that run wild, almost killing Douglass in the process.
Finally, Douglass has enough. Covey takes him to the barn, intending on beating him. Douglass fights back, however, with all his will. The fight lasts for two hours. After the fight, both men are exhausted. Covey leaves Douglass alone after the fight. Douglass emerges the victor; his spirit was not broken.