A protagonist in a story can be defined as the main character who battles the conflict in the story. Often, the battle is between the protagonist and the antagonist.
In Margaret Atwood's novel The Blind Assassin, one clue telling us the protagonist of the story is Iris Chase Griffen concerns the fact that the story is told from the first person point of view, and we know Iris is the narrator. First person point of view is achieved when a writer uses pronouns like I, me, and we to relay the story through the eyes of the character, making the reader one with the character. The character who is the I in the story is most often the main character. We see evidence of first person narration in the very first sentence of the book: "Ten days after the war ended, my sister Laura drove a car off a bride." Since there are only two sisters in the story--Irene and Laura--we know that Irene is the me in the story, which means the story is told from Irene's perspective, making her the main character.
Another clue she is the protagonist concerns the fact that she faces many challenges throughout the story while always remaining the focus of the story. Challenges she faces include winding up being married to a controlling, deceitful man and facing her sister's suicide.
The antagonist is defined as the enemy of the story; the antagonist is the one who creates the conflict and battles against the protagonist.
It can be said that Richard Griffen, Irene's husband, and his sister Winifred act as the antagonists of the story due to the number of conflicts they enter into with Irene and the amount of damage they inflict. Both Richard and Winifred are proven to be very manipulative, dictatorial characters. Irene recognizes the manipulative and dictatorial traits in Winifred when she first realizes Winifred controls all of Richard's affairs. Irene recognizes Richard as manipulative when she learns he lied to her about her father's factory closing and refused to tell her of her father's death while they were on their honeymoon. Later, Irene learns that Richard impregnated her sister Lauren, that he and Winifred had Lauren locked up as insane, and that Richard is the cause of Lauren's suicide.